<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e6a65-03be-70cf-90a4-476c37d86b3b,g:be6a23eb-92e9-40de-95cb-6df259e64259-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://casella.com/role/COVER" xlink:type="simple" xlink:href="cwst-20260630.xsd#COVER"/>
  <link:definitionLink xlink:role="http://casella.com/role/COVER" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019e6a65-05f9-71ac-bbf9-0ad70b4b5dbe" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentInformationTable_019e6a65-05f9-71ac-bbf9-0ad70b4b5dbe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-76c6-8cd7-f9e9f504f722" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019e6a65-05f9-71ac-bbf9-0ad70b4b5dbe" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-76c6-8cd7-f9e9f504f722" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-76c6-8cd7-f9e9f504f722_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-76c6-8cd7-f9e9f504f722" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-76c6-8cd7-f9e9f504f722_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-75ac-b975-9fc3a8cafe9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-76c6-8cd7-f9e9f504f722" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-75ac-b975-9fc3a8cafe9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7dbb-b598-f4c5a3fbf439" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-75ac-b975-9fc3a8cafe9e" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7dbb-b598-f4c5a3fbf439" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e6a65-05f9-7c63-9c4d-9e5f826917db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-75ac-b975-9fc3a8cafe9e" xlink:to="loc_us-gaap_CommonClassBMember_019e6a65-05f9-7c63-9c4d-9e5f826917db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019e6a65-05f9-7411-afd7-36942af7eb85" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentType_019e6a65-05f9-7411-afd7-36942af7eb85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019e6a65-05f9-79d4-864d-7c7ad6de2eb1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentQuarterlyReport_019e6a65-05f9-79d4-864d-7c7ad6de2eb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019e6a65-05f9-773b-8905-7350e25f2892" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentPeriodEndDate_019e6a65-05f9-773b-8905-7350e25f2892" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019e6a65-05f9-71b7-bb56-f6203edf609a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentTransitionReport_019e6a65-05f9-71b7-bb56-f6203edf609a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019e6a65-05f9-7d9d-a95d-9434370d888a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityFileNumber_019e6a65-05f9-7d9d-a95d-9434370d888a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019e6a65-05f9-7b5f-adfa-8b6c76472caf" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityRegistrantName_019e6a65-05f9-7b5f-adfa-8b6c76472caf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019e6a65-05f9-732e-9b78-b6353b66db42" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019e6a65-05f9-732e-9b78-b6353b66db42" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019e6a65-05f9-73e3-a3f4-1283a6883267" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityTaxIdentificationNumber_019e6a65-05f9-73e3-a3f4-1283a6883267" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019e6a65-05f9-737c-b55a-36b2ec070a06" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityAddressAddressLine1_019e6a65-05f9-737c-b55a-36b2ec070a06" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019e6a65-05f9-7bde-96df-c59eece4a30c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityAddressCityOrTown_019e6a65-05f9-7bde-96df-c59eece4a30c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019e6a65-05f9-7ca4-b482-1fbec46e1bc0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityAddressStateOrProvince_019e6a65-05f9-7ca4-b482-1fbec46e1bc0" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019e6a65-05f9-7919-a7ae-90e211afa29b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityAddressPostalZipCode_019e6a65-05f9-7919-a7ae-90e211afa29b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019e6a65-05f9-7dc1-a218-51a9f9a7b39e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_CityAreaCode_019e6a65-05f9-7dc1-a218-51a9f9a7b39e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019e6a65-05f9-7d7e-a523-4d2464883519" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_LocalPhoneNumber_019e6a65-05f9-7d7e-a523-4d2464883519" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019e6a65-05f9-75a2-a0da-54cde5e5eb16" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_Security12bTitle_019e6a65-05f9-75a2-a0da-54cde5e5eb16" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019e6a65-05f9-7d98-b118-b24d3ed44527" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_TradingSymbol_019e6a65-05f9-7d98-b118-b24d3ed44527" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019e6a65-05f9-71ab-a972-0d4bfa554612" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_SecurityExchangeName_019e6a65-05f9-71ab-a972-0d4bfa554612" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019e6a65-05f9-7a3e-945f-e03b9b5085e3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityCurrentReportingStatus_019e6a65-05f9-7a3e-945f-e03b9b5085e3" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019e6a65-05f9-7f6e-a0e7-0176017f3152" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityInteractiveDataCurrent_019e6a65-05f9-7f6e-a0e7-0176017f3152" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019e6a65-05f9-77fb-8fc5-177e7c5bb978" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityFilerCategory_019e6a65-05f9-77fb-8fc5-177e7c5bb978" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019e6a65-05f9-7ede-8ce0-a1227cf20f95" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntitySmallBusiness_019e6a65-05f9-7ede-8ce0-a1227cf20f95" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019e6a65-05f9-7e18-ba26-a8d64345ecc5" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityEmergingGrowthCompany_019e6a65-05f9-7e18-ba26-a8d64345ecc5" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019e6a65-05f9-78af-be0f-d870cec8c7e7" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityShellCompany_019e6a65-05f9-78af-be0f-d870cec8c7e7" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019e6a65-05f9-7e7f-b3cf-5a9be2af3636" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019e6a65-05f9-7e7f-b3cf-5a9be2af3636" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019e6a65-05f9-725c-a890-90200f13a93a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_EntityCentralIndexKey_019e6a65-05f9-725c-a890-90200f13a93a" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019e6a65-05f9-77d0-808a-9a7666e3a03c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_CurrentFiscalYearEndDate_019e6a65-05f9-77d0-808a-9a7666e3a03c" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019e6a65-05f9-7704-8cbb-e9bf0ab4f4f1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentFiscalYearFocus_019e6a65-05f9-7704-8cbb-e9bf0ab4f4f1" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019e6a65-05f9-789a-b094-1a080c69c96c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019e6a65-05f9-789a-b094-1a080c69c96c" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019e6a65-05f9-7430-996d-0dbc570ec4f5" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e6a65-05f9-72a8-ba02-5de3603b632b" xlink:to="loc_dei_AmendmentFlag_019e6a65-05f9-7430-996d-0dbc570ec4f5" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="cwst-20260630.xsd#CONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e6a65-05f9-7ba9-ab37-7f7cf022a2b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e6a65-05f9-7f02-8002-9d19784946df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7ba9-ab37-7f7cf022a2b4" xlink:to="loc_us-gaap_StatementTable_019e6a65-05f9-7f02-8002-9d19784946df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7cee-9dce-c05b590f9b04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e6a65-05f9-7f02-8002-9d19784946df" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7cee-9dce-c05b590f9b04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7cee-9dce-c05b590f9b04_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7cee-9dce-c05b590f9b04" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7cee-9dce-c05b590f9b04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7caf-ae04-cde08901e4eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7cee-9dce-c05b590f9b04" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7caf-ae04-cde08901e4eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7187-8de4-66c03f6cf6bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7caf-ae04-cde08901e4eb" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7187-8de4-66c03f6cf6bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e6a65-05f9-77a5-a53f-08f69bd95bce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7caf-ae04-cde08901e4eb" xlink:to="loc_us-gaap_CommonClassBMember_019e6a65-05f9-77a5-a53f-08f69bd95bce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7ba9-ab37-7f7cf022a2b4" xlink:to="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e6a65-05f9-773b-8ca2-e27ba23167cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e6a65-05f9-773b-8ca2-e27ba23167cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019e6a65-05f9-7bd9-bf4a-45cec837c1eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019e6a65-05f9-7bd9-bf4a-45cec837c1eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_019e6a65-05f9-793c-87a8-998e73bb4972" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_019e6a65-05f9-793c-87a8-998e73bb4972" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_019e6a65-05f9-7520-b715-961b4ad8b159" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:to="loc_us-gaap_OtherAssetsCurrent_019e6a65-05f9-7520-b715-961b4ad8b159" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019e6a65-05f9-716c-99f9-9b50530cbf2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e6a65-05f9-7c28-a25f-5ceace1ecbb9" xlink:to="loc_us-gaap_AssetsCurrent_019e6a65-05f9-716c-99f9-9b50530cbf2a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e6a65-05f9-7cf4-b712-115fba99adb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e6a65-05f9-7cf4-b712-115fba99adb7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e6a65-05f9-7924-9940-0443bf999d61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e6a65-05f9-7924-9940-0443bf999d61" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e6a65-05f9-7fcf-b3ce-2701c559e6a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_Goodwill_019e6a65-05f9-7fcf-b3ce-2701c559e6a2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e6a65-05f9-7e1f-9387-95bdf98f80f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e6a65-05f9-7e1f-9387-95bdf98f80f2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019e6a65-05f9-78bf-b713-7d7ef7de0bc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019e6a65-05f9-78bf-b713-7d7ef7de0bc9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_019e6a65-05f9-77ad-8029-95c361d8cfbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInvestmentsAndSecuritiesAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_019e6a65-05f9-77ad-8029-95c361d8cfbb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019e6a65-05f9-7da5-a57a-51b4fa064cf1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019e6a65-05f9-7da5-a57a-51b4fa064cf1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e6a65-05f9-762f-a308-39750c466b7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-05f9-7caf-a134-c70fbb744f43" xlink:to="loc_us-gaap_Assets_019e6a65-05f9-762f-a308-39750c466b7d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7ba9-ab37-7f7cf022a2b4" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019e6a65-05f9-7f4a-85e9-985ce2cb9198" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019e6a65-05f9-7f4a-85e9-985ce2cb9198" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_019e6a65-05f9-7f52-b994-f89413af32b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_019e6a65-05f9-7f52-b994-f89413af32b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019e6a65-05f9-7fea-9332-21d88d6a2a58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_AccountsPayableCurrent_019e6a65-05f9-7fea-9332-21d88d6a2a58" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019e6a65-05f9-7ec2-854f-631afd03070f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019e6a65-05f9-7ec2-854f-631afd03070f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e6a65-05f9-7683-b122-40216457525b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e6a65-05f9-7683-b122-40216457525b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_019e6a65-05f9-7f66-ae9e-f7d2c75dfbc4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_019e6a65-05f9-7f66-ae9e-f7d2c75dfbc4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019e6a65-05f9-7a64-a123-aa78b2366b84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019e6a65-05f9-7a64-a123-aa78b2366b84" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019e6a65-05f9-75f3-b855-f56dcdde49f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e6a65-05f9-7eb4-b3d4-7ac60c8b3875" xlink:to="loc_us-gaap_LiabilitiesCurrent_019e6a65-05f9-75f3-b855-f56dcdde49f0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019e6a65-05f9-76c9-863d-f38ba956716f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019e6a65-05f9-76c9-863d-f38ba956716f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019e6a65-05f9-7170-9837-680ef547853a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019e6a65-05f9-7170-9837-680ef547853a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_019e6a65-05f9-7db2-85ce-dfd13abf3fe6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_019e6a65-05f9-7db2-85ce-dfd13abf3fe6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019e6a65-05f9-7109-a459-35a0937b37c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019e6a65-05f9-7109-a459-35a0937b37c5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_019e6a65-05f9-72fc-9958-bddef1b2419d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_019e6a65-05f9-72fc-9958-bddef1b2419d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_019e6a65-05f9-7191-8942-04a200f1b730" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_CommitmentsAndContingencies_019e6a65-05f9-7191-8942-04a200f1b730" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019e6a65-05f9-72e6-9a17-ca1db19f27c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:to="loc_us-gaap_CommonStockValue_019e6a65-05f9-72e6-9a17-ca1db19f27c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019e6a65-05f9-75c3-9cf4-1997b993fbc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019e6a65-05f9-75c3-9cf4-1997b993fbc0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019e6a65-05f9-72fb-9fbd-a5331e2c16d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019e6a65-05f9-72fb-9fbd-a5331e2c16d1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-05f9-7ca2-be90-cffe92c97b94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-05f9-7ca2-be90-cffe92c97b94" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e6a65-05f9-7707-af20-0bb074ce1e3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019e6a65-05f9-711b-ba80-5e58f5fbbcd0" xlink:to="loc_us-gaap_StockholdersEquity_019e6a65-05f9-7707-af20-0bb074ce1e3e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019e6a65-05f9-7f9e-9192-fc3c97817794" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e6a65-05f9-72d0-8e33-5f3198f9ae93" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019e6a65-05f9-7f9e-9192-fc3c97817794" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="cwst-20260630.xsd#CONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e6a65-05f9-757e-985c-96fb622ae6c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_StatementTable_019e6a65-05f9-757e-985c-96fb622ae6c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7462-bfca-20f38c405c75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e6a65-05f9-757e-985c-96fb622ae6c7" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7462-bfca-20f38c405c75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7462-bfca-20f38c405c75_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7462-bfca-20f38c405c75" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-7462-bfca-20f38c405c75_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-748e-b6b2-143ab80db0e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-05f9-7462-bfca-20f38c405c75" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-748e-b6b2-143ab80db0e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7510-9faf-5730abfc7075" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-748e-b6b2-143ab80db0e7" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-05f9-7510-9faf-5730abfc7075" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e6a65-05f9-7291-be0d-7d5b73c8c4ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-05f9-748e-b6b2-143ab80db0e7" xlink:to="loc_us-gaap_CommonClassBMember_019e6a65-05f9-7291-be0d-7d5b73c8c4ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019e6a65-05f9-7d2f-9935-ef5b17942b1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019e6a65-05f9-7d2f-9935-ef5b17942b1c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019e6a65-05f9-7dda-a716-e9a758acb023" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019e6a65-05f9-7dda-a716-e9a758acb023" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e6a65-05f9-78db-a9d3-82ad8576d36b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e6a65-05f9-78db-a9d3-82ad8576d36b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019e6a65-05f9-77f9-a3ef-e4f7f8637070" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019e6a65-05f9-77f9-a3ef-e4f7f8637070" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019e6a65-05f9-7c60-aa1d-ade1d73e47f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_CommonStockSharesIssued_019e6a65-05f9-7c60-aa1d-ade1d73e47f8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e6a65-05f9-78cc-9836-feb7087219c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e6a65-05f9-78cc-9836-feb7087219c5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_StockholdersEquityNumberOfVotesEachShare_019e6a65-05f9-7bfc-9020-9ae75b52340a" xlink:href="cwst-20260630.xsd#cwst_StockholdersEquityNumberOfVotesEachShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-05f9-7d95-a001-5ecd35aae608" xlink:to="loc_cwst_StockholdersEquityNumberOfVotesEachShare_019e6a65-05f9-7bfc-9020-9ae75b52340a" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="cwst-20260630.xsd#UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://casella.com/role/UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e6a65-06cd-79a2-b43f-27e6f1e874a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e6a65-06cd-779c-9c05-1aa3f99e4c86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-06cd-79a2-b43f-27e6f1e874a9" xlink:to="loc_us-gaap_StatementTable_019e6a65-06cd-779c-9c05-1aa3f99e4c86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7b17-a166-113dc5c87842" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e6a65-06cd-779c-9c05-1aa3f99e4c86" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7b17-a166-113dc5c87842" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7b17-a166-113dc5c87842_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7b17-a166-113dc5c87842" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7b17-a166-113dc5c87842_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7c6b-b56e-657385987dc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7b17-a166-113dc5c87842" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7c6b-b56e-657385987dc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-06cd-71b6-99b4-d6e26c7a7f8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7c6b-b56e-657385987dc5" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-06cd-71b6-99b4-d6e26c7a7f8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e6a65-06cd-736c-8d43-a173b26f97a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7c6b-b56e-657385987dc5" xlink:to="loc_us-gaap_CommonClassBMember_019e6a65-06cd-736c-8d43-a173b26f97a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-70b9-af4c-86f2932d90a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e6a65-06cd-779c-9c05-1aa3f99e4c86" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-70b9-af4c-86f2932d90a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-70b9-af4c-86f2932d90a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-70b9-af4c-86f2932d90a6" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-70b9-af4c-86f2932d90a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-70b9-af4c-86f2932d90a6" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e6a65-06cd-716a-9ca3-27f70612cb58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:to="loc_us-gaap_CommonStockMember_019e6a65-06cd-716a-9ca3-27f70612cb58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e6a65-06cd-79e2-b555-3c0367bbce1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e6a65-06cd-79e2-b555-3c0367bbce1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e6a65-06cd-718f-90e5-e1d66054040f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:to="loc_us-gaap_RetainedEarningsMember_019e6a65-06cd-718f-90e5-e1d66054040f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e6a65-06cd-78ea-ac27-4eb7f2fa82fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-79e6-a5cf-32527be1b7c2" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e6a65-06cd-78ea-ac27-4eb7f2fa82fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6a65-06cd-79a2-b43f-27e6f1e874a9" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e6a65-06cd-71ab-8326-b581693ae0c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_StockholdersEquity_019e6a65-06cd-71ab-8326-b581693ae0c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e6a65-06cd-7eb6-bd14-e2c097e119cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e6a65-06cd-7eb6-bd14-e2c097e119cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e6a65-06cd-7a97-86a3-edd5f58d1cbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e6a65-06cd-7a97-86a3-edd5f58d1cbc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e6a65-06cd-7a5b-9c2b-60fe7d4dab02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e6a65-06cd-7a5b-9c2b-60fe7d4dab02" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e6a65-06cd-7d5d-a14e-f36340689c04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e6a65-06cd-7d5d-a14e-f36340689c04" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019e6a65-06cd-7122-b54d-9da04c983a02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6a65-06cd-71f9-a8dc-faf103d32122" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019e6a65-06cd-7122-b54d-9da04c983a02" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e6a65-06cd-78f5-bd98-5d7d5bed20c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019e6a65-06cd-7122-b54d-9da04c983a02" xlink:to="loc_us-gaap_NetIncomeLoss_019e6a65-06cd-78f5-bd98-5d7d5bed20c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019e6a65-06cd-7a6d-b1f5-0ed5ab4302b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019e6a65-06cd-7122-b54d-9da04c983a02" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019e6a65-06cd-7a6d-b1f5-0ed5ab4302b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e6a65-06cd-7940-a641-ad5d69dd4e37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019e6a65-06cd-7a6d-b1f5-0ed5ab4302b9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e6a65-06cd-7940-a641-ad5d69dd4e37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e6a65-06cd-7348-9335-1f1eb64ec960" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e6a65-06cd-717c-9e0b-64b1b71cdd23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#BASISOFPRESENTATIONDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-72b6-90ad-b37ece817e24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7ce2-8ca7-46b6b7ef8c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-72b6-90ad-b37ece817e24" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7ce2-8ca7-46b6b7ef8c54" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7c11-ad63-ab0962190ab0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7ce2-8ca7-46b6b7ef8c54" xlink:to="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7c11-ad63-ab0962190ab0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7c11-ad63-ab0962190ab0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7c11-ad63-ab0962190ab0" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7c11-ad63-ab0962190ab0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7cff-9192-e46b25f88069" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7c11-ad63-ab0962190ab0" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7cff-9192-e46b25f88069" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-767c-8933-71b193840337" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7cff-9192-e46b25f88069" xlink:to="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-767c-8933-71b193840337" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfStatesInWhichEntityOperates_019e6a65-06cd-7f7a-88ac-eb1d966b57ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfStatesInWhichEntityOperates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-72b6-90ad-b37ece817e24" xlink:to="loc_us-gaap_NumberOfStatesInWhichEntityOperates_019e6a65-06cd-7f7a-88ac-eb1d966b57ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019e6a65-06cd-7a31-be1c-82cd03d485f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-72b6-90ad-b37ece817e24" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019e6a65-06cd-7a31-be1c-82cd03d485f7" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7743-83ba-8912eeec6e11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-77d4-9b3f-02766059a12a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7743-83ba-8912eeec6e11" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-77d4-9b3f-02766059a12a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7948-b570-9f36ec1bfecd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-77d4-9b3f-02766059a12a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7948-b570-9f36ec1bfecd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-7948-b570-9f36ec1bfecd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7948-b570-9f36ec1bfecd" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-7948-b570-9f36ec1bfecd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7948-b570-9f36ec1bfecd" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019e6a65-06cd-763c-953e-0419b7cdcbb2" xlink:href="cwst-20260630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:to="loc_cwst_EasternRegionMember_019e6a65-06cd-763c-953e-0419b7cdcbb2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019e6a65-06cd-7cec-b9ba-b87f7a29824f" xlink:href="cwst-20260630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:to="loc_cwst_WesternRegionMember_019e6a65-06cd-7cec-b9ba-b87f7a29824f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7c94-8392-4f43b09a598f" xlink:href="cwst-20260630.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:to="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7c94-8392-4f43b09a598f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-7642-a47b-96953f1b5b72" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-789a-b5ba-2352c4ae0e78" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-7642-a47b-96953f1b5b72" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7f4c-b532-d719722fc318" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-77d4-9b3f-02766059a12a" xlink:to="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7f4c-b532-d719722fc318" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7f4c-b532-d719722fc318_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7f4c-b532-d719722fc318" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7f4c-b532-d719722fc318_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7f4c-b532-d719722fc318" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CollectionMember_019e6a65-06cd-7831-bf1e-3f2765cf3639" xlink:href="cwst-20260630.xsd#cwst_CollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_CollectionMember_019e6a65-06cd-7831-bf1e-3f2765cf3639" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillRevenueMember_019e6a65-06cd-74d9-b469-21556c4643c7" xlink:href="cwst-20260630.xsd#cwst_LandfillRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_LandfillRevenueMember_019e6a65-06cd-74d9-b469-21556c4643c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationMember_019e6a65-06cd-73a0-9eda-62b0f880b593" xlink:href="cwst-20260630.xsd#cwst_TransferStationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_TransferStationMember_019e6a65-06cd-73a0-9eda-62b0f880b593" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransportationMember_019e6a65-06cd-70b3-bbc0-679e0b4f72a2" xlink:href="cwst-20260630.xsd#cwst_TransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_TransportationMember_019e6a65-06cd-70b3-bbc0-679e0b4f72a2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillGasToEnergyMember_019e6a65-06cd-71ee-b356-06a51fff59d4" xlink:href="cwst-20260630.xsd#cwst_LandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_LandfillGasToEnergyMember_019e6a65-06cd-71ee-b356-06a51fff59d4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ProcessingServicesMember_019e6a65-06cd-799c-b137-5176f46e4dc5" xlink:href="cwst-20260630.xsd#cwst_ProcessingServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_ProcessingServicesMember_019e6a65-06cd-799c-b137-5176f46e4dc5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_019e6a65-06cd-79c7-8e5e-41067e0a37ce" xlink:href="cwst-20260630.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-77ae-85fd-66e0d89d7dee" xlink:to="loc_cwst_NationalAccountsMember_019e6a65-06cd-79c7-8e5e-41067e0a37ce" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e6a65-06cd-71fe-9ed1-d361222d503a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-77d4-9b3f-02766059a12a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e6a65-06cd-71fe-9ed1-d361222d503a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-71fe-9ed1-d361222d503a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e6a65-06cd-71fe-9ed1-d361222d503a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-71fe-9ed1-d361222d503a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-7483-96d5-be8299f1a7df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e6a65-06cd-71fe-9ed1-d361222d503a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-7483-96d5-be8299f1a7df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019e6a65-06cd-70e3-a28e-b639b8e5b02f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-7483-96d5-be8299f1a7df" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019e6a65-06cd-70e3-a28e-b639b8e5b02f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019e6a65-06cd-7ac0-8da5-c4246359bfb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e6a65-06cd-7483-96d5-be8299f1a7df" xlink:to="loc_us-gaap_TransferredOverTimeMember_019e6a65-06cd-7ac0-8da5-c4246359bfb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6a65-06cd-75d6-9d59-f5cc85a040ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7743-83ba-8912eeec6e11" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6a65-06cd-75d6-9d59-f5cc85a040ec" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#REVENUERECOGNITIONNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-7a40-81d6-0ab034570964" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-7a40-81d6-0ab034570964" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6a65-06cd-70ae-920b-aee2c24023a4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e6a65-06cd-7a40-81d6-0ab034570964" xlink:to="loc_srt_ProductOrServiceAxis_019e6a65-06cd-70ae-920b-aee2c24023a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-70ae-920b-aee2c24023a4_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-70ae-920b-aee2c24023a4" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-70ae-920b-aee2c24023a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7c46-bb2d-806d5f5edc94" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-70ae-920b-aee2c24023a4" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7c46-bb2d-806d5f5edc94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_019e6a65-06cd-7586-9096-638411f7abf0" xlink:href="cwst-20260630.xsd#cwst_RebateRecycledOrReturnedOrganicMaterialsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7c46-bb2d-806d5f5edc94" xlink:to="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_019e6a65-06cd-7586-9096-638411f7abf0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6a65-06cd-7f65-9443-24a08daa775b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6a65-06cd-7f65-9443-24a08daa775b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_019e6a65-06cd-731d-bba3-483b1e60572e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:to="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_019e6a65-06cd-731d-bba3-483b1e60572e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_019e6a65-06cd-7033-8b1c-56d845ae787d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_019e6a65-06cd-7033-8b1c-56d845ae787d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e6a65-06cd-7d8b-b59f-d700e9533eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e6a65-06cd-7e10-a5c6-df7d3a790e0a" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e6a65-06cd-7d8b-b59f-d700e9533eb3" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#BUSINESSCOMBINATIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7adc-825d-9e92e6e1bba7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7878-b2e7-52a6692435d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7adc-825d-9e92e6e1bba7" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7878-b2e7-52a6692435d8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7e66-add2-051fc5309697" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7878-b2e7-52a6692435d8" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7e66-add2-051fc5309697" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-7e66-add2-051fc5309697_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7e66-add2-051fc5309697" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-7e66-add2-051fc5309697_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-7acd-9ee8-7f806a18da40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-7e66-add2-051fc5309697" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-7acd-9ee8-7f806a18da40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019f145d-5e78-7ef7-99fb-4255e00fda71" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7acd-9ee8-7f806a18da40" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019f145d-5e78-7ef7-99fb-4255e00fda71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019f145d-5e78-7830-8675-759fb4a89e70" xlink:href="cwst-20260630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7acd-9ee8-7f806a18da40" xlink:to="loc_cwst_WesternRegionMember_019f145d-5e78-7830-8675-759fb4a89e70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7d19-977e-073eedde3e87" xlink:href="cwst-20260630.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7acd-9ee8-7f806a18da40" xlink:to="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7d19-977e-073eedde3e87" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6a65-06cd-77ed-ab6c-8455fa21c1dc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7878-b2e7-52a6692435d8" xlink:to="loc_srt_RangeAxis_019e6a65-06cd-77ed-ab6c-8455fa21c1dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-77ed-ab6c-8455fa21c1dc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-77ed-ab6c-8455fa21c1dc" xlink:to="loc_srt_RangeMember_019e6a65-06cd-77ed-ab6c-8455fa21c1dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-7f13-9b74-71bf6669e9bc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-77ed-ab6c-8455fa21c1dc" xlink:to="loc_srt_RangeMember_019e6a65-06cd-7f13-9b74-71bf6669e9bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6a65-06cd-7990-a862-ea2ddd94214e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-7f13-9b74-71bf6669e9bc" xlink:to="loc_srt_MinimumMember_019e6a65-06cd-7990-a862-ea2ddd94214e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6a65-06cd-7293-a85b-177f7ae96d31" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-7f13-9b74-71bf6669e9bc" xlink:to="loc_srt_MaximumMember_019e6a65-06cd-7293-a85b-177f7ae96d31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019f145b-908b-7956-845f-12a4a5263864" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7878-b2e7-52a6692435d8" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019f145b-908b-7956-845f-12a4a5263864" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f145b-908b-7956-845f-12a4a5263864_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019f145b-908b-7956-845f-12a4a5263864" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f145b-908b-7956-845f-12a4a5263864_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f145b-908b-73f9-a652-e93e5841ea4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019f145b-908b-7956-845f-12a4a5263864" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f145b-908b-73f9-a652-e93e5841ea4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TuckInOperationBusinessesMember_019f145c-724c-7dcc-bd40-141b854c23fd" xlink:href="cwst-20260630.xsd#cwst_TuckInOperationBusinessesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f145b-908b-73f9-a652-e93e5841ea4e" xlink:to="loc_cwst_TuckInOperationBusinessesMember_019f145c-724c-7dcc-bd40-141b854c23fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_019e6a65-06cd-7174-aac0-894081445907" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfBusinessesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7adc-825d-9e92e6e1bba7" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_019e6a65-06cd-7174-aac0-894081445907" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e6a65-06cd-7e7f-a58f-ac29a18310df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7adc-825d-9e92e6e1bba7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e6a65-06cd-7e7f-a58f-ac29a18310df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019fa700-a97b-7267-9a4a-07f1cd323d9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7adc-825d-9e92e6e1bba7" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019fa700-a97b-7267-9a4a-07f1cd323d9a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7d9a-b21c-4bdfbd2e9eb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7d9a-b21c-4bdfbd2e9eb9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019e6a65-06cd-78a8-816e-c98ee7604636" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7d9a-b21c-4bdfbd2e9eb9" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019e6a65-06cd-78a8-816e-c98ee7604636" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e6a65-06cd-78a8-816e-c98ee7604636_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e6a65-06cd-78a8-816e-c98ee7604636" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e6a65-06cd-78a8-816e-c98ee7604636_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e6a65-06cd-7c7f-9273-82f871de5c36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e6a65-06cd-78a8-816e-c98ee7604636" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e6a65-06cd-7c7f-9273-82f871de5c36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationsAcquisitionMember_019e6a65-06cd-76fb-9d0d-45c713a26f87" xlink:href="cwst-20260630.xsd#cwst_TransferStationsAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e6a65-06cd-7c7f-9273-82f871de5c36" xlink:to="loc_cwst_TransferStationsAcquisitionMember_019e6a65-06cd-76fb-9d0d-45c713a26f87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-72e6-b970-eccb43b5f06c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e6a65-06cd-7d9a-b21c-4bdfbd2e9eb9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-72e6-b970-eccb43b5f06c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-72e6-b970-eccb43b5f06c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-72e6-b970-eccb43b5f06c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-72e6-b970-eccb43b5f06c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a3e-a7a8-1046926f9bc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-72e6-b970-eccb43b5f06c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a3e-a7a8-1046926f9bc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019e6a65-06cd-7c18-8557-b5d52211adf8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a3e-a7a8-1046926f9bc7" xlink:to="loc_us-gaap_TradeNamesMember_019e6a65-06cd-7c18-8557-b5d52211adf8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019e6a65-06cd-7ea3-b66d-4b6143e1eece" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a3e-a7a8-1046926f9bc7" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019e6a65-06cd-7ea3-b66d-4b6143e1eece" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019e6a65-06cd-7915-97c5-ca927db59291" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a3e-a7a8-1046926f9bc7" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019e6a65-06cd-7915-97c5-ca927db59291" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019e6a65-06cd-750c-8bae-b173abc808e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019e6a65-06cd-750c-8bae-b173abc808e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAcquiredFromAcquisition_019e6a65-06cd-71cc-89b7-3fcc6cff5834" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAcquiredFromAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019e6a65-06cd-750c-8bae-b173abc808e9" xlink:to="loc_us-gaap_CashAcquiredFromAcquisition_019e6a65-06cd-71cc-89b7-3fcc6cff5834" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements_019e6a65-06cd-79fd-8423-f9b5b1e6e3e3" xlink:href="cwst-20260630.xsd#cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:to="loc_cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements_019e6a65-06cd-79fd-8423-f9b5b1e6e3e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_019f03f4-71d2-705d-b722-73d689f09917" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_019f03f4-71d2-705d-b722-73d689f09917" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019e6a65-06cd-73f9-818e-2e3c3976d091" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019e6a65-06cd-7ff7-b63a-0ae79365590c" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019e6a65-06cd-73f9-818e-2e3c3976d091" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_019e6a65-06cd-7eec-914e-b585cf1e4ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_019e6a65-06cd-7eec-914e-b585cf1e4ca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_019e6a65-06cd-745e-996a-c0def57585cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_019e6a65-06cd-745e-996a-c0def57585cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_019e6a65-06cd-7ceb-86a6-10044819b914" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_019e6a65-06cd-7ceb-86a6-10044819b914" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019e6a65-06cd-7d6b-8bd4-952a2a3d31a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019e6a65-06cd-7d6b-8bd4-952a2a3d31a1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_019e6a65-06cd-7121-a273-78290038461a" xlink:href="cwst-20260630.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_019e6a65-06cd-7121-a273-78290038461a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019e6a65-06cd-766e-8e3d-251cd741ab68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019e6a65-06cd-766e-8e3d-251cd741ab68" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019f5d19-edd6-7eac-86c2-cb0a828a3245" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019f5d19-edd6-7eac-86c2-cb0a828a3245" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_019e6a65-06cd-710b-a7ca-820a0d1d8e95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_019e6a65-06cd-710b-a7ca-820a0d1d8e95" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_019e6a65-06cd-7bc3-a402-9f2525bf0f2b" xlink:href="cwst-20260630.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_019e6a65-06cd-7bc3-a402-9f2525bf0f2b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019e6a65-06cd-7f41-8c60-6f16987b86e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019e6a65-06cd-77fe-bef9-31c59bd318c3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019e6a65-06cd-7f41-8c60-6f16987b86e2" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e6a65-06cd-7d5a-8448-0b52795a85df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:to="loc_us-gaap_Goodwill_019e6a65-06cd-7d5a-8448-0b52795a85df" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019e6a65-06cd-74f2-b562-6758999ba368" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e6a65-06cd-7b3e-bfbf-b1e889ddd360" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019e6a65-06cd-74f2-b562-6758999ba368" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_019e6a65-06cd-78dc-b1c5-e4bb6e837870" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_019e6a65-06cd-73e7-bf05-87d08f8ded04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_019e6a65-06cd-78dc-b1c5-e4bb6e837870" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_019e6a65-06cd-73e7-bf05-87d08f8ded04" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-71f0-87d8-1e6ac4e3e940" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_019e6a65-06cd-73e7-bf05-87d08f8ded04" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-71f0-87d8-1e6ac4e3e940" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-71f0-87d8-1e6ac4e3e940_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-71f0-87d8-1e6ac4e3e940" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-71f0-87d8-1e6ac4e3e940_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-71f0-87d8-1e6ac4e3e940" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019e6a65-06cd-7cf9-9bb8-f4935caeca04" xlink:href="cwst-20260630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:to="loc_cwst_EasternRegionMember_019e6a65-06cd-7cf9-9bb8-f4935caeca04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019e6a65-06cd-7f3e-b8f3-897dabf7f96d" xlink:href="cwst-20260630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:to="loc_cwst_WesternRegionMember_019e6a65-06cd-7f3e-b8f3-897dabf7f96d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7b3b-a43c-c01c840fe61b" xlink:href="cwst-20260630.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:to="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7b3b-a43c-c01c840fe61b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-7c39-bf19-5637a375ff78" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-7a4c-86fc-ba1d68a0f944" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-7c39-bf19-5637a375ff78" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_019e6a65-06cd-77ec-9864-7a96c50f4446" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019e6a65-06cd-78dc-b1c5-e4bb6e837870" xlink:to="loc_us-gaap_GoodwillRollForward_019e6a65-06cd-77ec-9864-7a96c50f4446" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e6a65-06cd-72df-bc9b-c7261d932f42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019e6a65-06cd-77ec-9864-7a96c50f4446" xlink:to="loc_us-gaap_Goodwill_019e6a65-06cd-72df-bc9b-c7261d932f42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_019e6a65-06cd-7cc0-871f-af92b752a7d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019e6a65-06cd-77ec-9864-7a96c50f4446" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_019e6a65-06cd-7cc0-871f-af92b752a7d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019e6a65-06cd-7afe-b3df-0d306670fc1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019e6a65-06cd-77ec-9864-7a96c50f4446" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019e6a65-06cd-7afe-b3df-0d306670fc1a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e6a65-06cd-7190-a360-e74b09919702" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e6a65-06cd-75ff-9385-81d56109acf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e6a65-06cd-778e-a6ba-7cb1c1615f92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e6a65-06cd-75ff-9385-81d56109acf6" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e6a65-06cd-778e-a6ba-7cb1c1615f92" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-7453-b235-f0db4dde449e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e6a65-06cd-778e-a6ba-7cb1c1615f92" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-7453-b235-f0db4dde449e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7453-b235-f0db4dde449e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-7453-b235-f0db4dde449e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7453-b235-f0db4dde449e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a63-9c53-bcc9565fc03e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e6a65-06cd-7453-b235-f0db4dde449e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a63-9c53-bcc9565fc03e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019e6a65-06cd-7fe5-85b8-476a307a4ee6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a63-9c53-bcc9565fc03e" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019e6a65-06cd-7fe5-85b8-476a307a4ee6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019e6a65-06cd-7acc-89b1-216428bfc3c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a63-9c53-bcc9565fc03e" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019e6a65-06cd-7acc-89b1-216428bfc3c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019e6a65-06cd-718a-90f4-d69fac7072be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e6a65-06cd-7a63-9c53-bcc9565fc03e" xlink:to="loc_us-gaap_TradeNamesMember_019e6a65-06cd-718a-90f4-d69fac7072be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e6a65-06cd-7fe8-b756-f729ac6fd072" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e6a65-06cd-75ff-9385-81d56109acf6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e6a65-06cd-7fe8-b756-f729ac6fd072" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e6a65-06cd-7adb-b842-b630d6fc9739" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e6a65-06cd-75ff-9385-81d56109acf6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e6a65-06cd-7adb-b842-b630d6fc9739" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e6a65-06cd-7e3a-828f-4c535ebad539" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e6a65-06cd-75ff-9385-81d56109acf6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e6a65-06cd-7e3a-828f-4c535ebad539" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofDebtDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTScheduleofDebtDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7b86-91e5-c36e2fb50969" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7b86-91e5-c36e2fb50969" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7cec-8401-158b7e81f037" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7b86-91e5-c36e2fb50969" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7cec-8401-158b7e81f037" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7cec-8401-158b7e81f037_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7cec-8401-158b7e81f037" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7cec-8401-158b7e81f037_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7cec-8401-158b7e81f037" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7aad-98e5-0e889842301e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:to="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7aad-98e5-0e889842301e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-74e5-ba59-f00df6df07dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:to="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-74e5-ba59-f00df6df07dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7c7b-ab13-5084e236a114" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:to="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7c7b-ab13-5084e236a114" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NotesPayableMember_019e6a65-06cd-7f8d-839c-5a35530957ba" xlink:href="cwst-20260630.xsd#cwst_NotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7db3-b04d-f65542350a28" xlink:to="loc_cwst_NotesPayableMember_019e6a65-06cd-7f8d-839c-5a35530957ba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-76e2-93bc-8bafeffcccc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7b86-91e5-c36e2fb50969" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-76e2-93bc-8bafeffcccc7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-76e2-93bc-8bafeffcccc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-76e2-93bc-8bafeffcccc7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-76e2-93bc-8bafeffcccc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-76e2-93bc-8bafeffcccc7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermLoanAFacilityDueSeptember2029Member_019e6a65-06cd-726b-b822-982a75c4a85d" xlink:href="cwst-20260630.xsd#cwst_TermLoanAFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_TermLoanAFacilityDueSeptember2029Member_019e6a65-06cd-726b-b822-982a75c4a85d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_019e6a65-06cd-7183-bae6-8eac4d187ba3" xlink:href="cwst-20260630.xsd#cwst_RevolvingCreditFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_019e6a65-06cd-7183-bae6-8eac4d187ba3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_019e6a65-06cd-78b1-a85b-20403938f7a9" xlink:href="cwst-20260630.xsd#cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_019e6a65-06cd-78b1-a85b-20403938f7a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019e6a65-06cd-7a12-9b5f-642df9e84904" xlink:href="cwst-20260630.xsd#cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019e6a65-06cd-7a12-9b5f-642df9e84904" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_019e6a65-06cd-7824-b624-ebc39ca10c37" xlink:href="cwst-20260630.xsd#cwst_RevenueBondsSeries2020DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_019e6a65-06cd-7824-b624-ebc39ca10c37" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_019e6a65-06cd-71b9-8780-b8f0166cfff5" xlink:href="cwst-20260630.xsd#cwst_RevenueBondsSeries2020R2DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_019e6a65-06cd-71b9-8780-b8f0166cfff5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member_019e6a65-06cd-760c-bc73-72adcbbd1861" xlink:href="cwst-20260630.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member_019e6a65-06cd-760c-bc73-72adcbbd1861" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_019e6a65-06cd-737e-b996-bbaef09b77bc" xlink:href="cwst-20260630.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_019e6a65-06cd-737e-b996-bbaef09b77bc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_019e6a65-06cd-7c36-bc00-a778bd718e1f" xlink:href="cwst-20260630.xsd#cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_019e6a65-06cd-7c36-bc00-a778bd718e1f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A1Due2052Member_019e6a65-06cd-7a47-ad41-673b77d94136" xlink:href="cwst-20260630.xsd#cwst_VermontBondsSeries2022A1Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_VermontBondsSeries2022A1Due2052Member_019e6a65-06cd-7a47-ad41-673b77d94136" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_019e6a65-06cd-7756-a159-a6d048728acc" xlink:href="cwst-20260630.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_019e6a65-06cd-7756-a159-a6d048728acc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_019e6a65-06cd-75cd-b20c-016dd98c3ad0" xlink:href="cwst-20260630.xsd#cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7530-a287-985308db6188" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_019e6a65-06cd-75cd-b20c-016dd98c3ad0" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e6a65-06cd-7606-87f7-f2ef46bec911" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e6a65-06cd-7606-87f7-f2ef46bec911" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6a65-06cd-7b83-af03-dfb7fe58efdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6a65-06cd-7b83-af03-dfb7fe58efdb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019e6a65-06cd-7a83-b9a4-7b8302408308" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019e6a65-06cd-7a83-b9a4-7b8302408308" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e6a65-06cd-77a8-a94d-0991939eaedf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e6a65-06cd-77a8-a94d-0991939eaedf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019e6a65-06cd-7e69-8e0b-20edb3848f20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_FinanceLeaseLiability_019e6a65-06cd-7e69-8e0b-20edb3848f20" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019e6a65-06cd-7f85-9c92-2207f28be5ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseLiability_019e6a65-06cd-7e69-8e0b-20edb3848f20" xlink:to="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019e6a65-06cd-7f85-9c92-2207f28be5ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_019e6a65-06cd-7286-a0dd-2ac35e5646e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_019e6a65-06cd-7286-a0dd-2ac35e5646e9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019e6a65-06cd-7e45-8e4c-d8e20e993a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019e6a65-06cd-7e45-8e4c-d8e20e993a3c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019e6a65-06cd-7941-8b3f-46dd2ae0e1e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019e6a65-06cd-7941-8b3f-46dd2ae0e1e4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019e6a65-06cd-7738-ae2e-9e33ea6ca154" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019e6a65-06cd-7738-ae2e-9e33ea6ca154" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019e6a65-06cd-7428-907d-87f625b2ed07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c19-877b-664a6e940b62" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019e6a65-06cd-7428-907d-87f625b2ed07" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCreditFacilityDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTNarrativeCreditFacilityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeCreditFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-756a-8acf-5e2945dada12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-756a-8acf-5e2945dada12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-756a-8acf-5e2945dada12_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-756a-8acf-5e2945dada12" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-756a-8acf-5e2945dada12_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7ebd-9274-6810afb2d2e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-756a-8acf-5e2945dada12" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7ebd-9274-6810afb2d2e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_019e6a65-06cd-79c1-8d98-b1273d7c0620" xlink:href="cwst-20260630.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7ebd-9274-6810afb2d2e4" xlink:to="loc_cwst_CreditFacilityMember_019e6a65-06cd-79c1-8d98-b1273d7c0620" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7f62-b000-a7844aab8b69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7f62-b000-a7844aab8b69" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7f62-b000-a7844aab8b69_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7f62-b000-a7844aab8b69" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7f62-b000-a7844aab8b69_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7431-b569-b2b5451ca6df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-7f62-b000-a7844aab8b69" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7431-b569-b2b5451ca6df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7efe-ad93-a95d7d08eda2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7431-b569-b2b5451ca6df" xlink:to="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7efe-ad93-a95d7d08eda2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-7b96-a1c3-afe738d21e47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7431-b569-b2b5451ca6df" xlink:to="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-7b96-a1c3-afe738d21e47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e6a65-06cd-7b6a-8216-f635786d092d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:to="loc_us-gaap_CreditFacilityAxis_019e6a65-06cd-7b6a-8216-f635786d092d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7b6a-8216-f635786d092d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e6a65-06cd-7b6a-8216-f635786d092d" xlink:to="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7b6a-8216-f635786d092d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7fad-ac01-ac568b12127d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e6a65-06cd-7b6a-8216-f635786d092d" xlink:to="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7fad-ac01-ac568b12127d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e6a65-06cd-7a44-a62d-49c5a484abe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7fad-ac01-ac568b12127d" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e6a65-06cd-7a44-a62d-49c5a484abe9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019e6a65-06cd-74bf-8d44-67165a39f934" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6a65-06cd-7fad-ac01-ac568b12127d" xlink:to="loc_us-gaap_LetterOfCreditMember_019e6a65-06cd-74bf-8d44-67165a39f934" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6a65-06cd-7b64-8a55-dc5db5064de1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019e6a65-06cd-753c-ab66-0245ad1fe11f" xlink:to="loc_srt_RangeAxis_019e6a65-06cd-7b64-8a55-dc5db5064de1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-7b64-8a55-dc5db5064de1_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7b64-8a55-dc5db5064de1" xlink:to="loc_srt_RangeMember_019e6a65-06cd-7b64-8a55-dc5db5064de1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-70a9-b4bd-72c12c31f158" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7b64-8a55-dc5db5064de1" xlink:to="loc_srt_RangeMember_019e6a65-06cd-70a9-b4bd-72c12c31f158" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6a65-06cd-70d1-8fd7-c2e064500632" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-70a9-b4bd-72c12c31f158" xlink:to="loc_srt_MinimumMember_019e6a65-06cd-70d1-8fd7-c2e064500632" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6a65-06cd-7c12-b426-517f94cf9a31" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-70a9-b4bd-72c12c31f158" xlink:to="loc_srt_MaximumMember_019e6a65-06cd-7c12-b426-517f94cf9a31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e6a65-06cd-7d67-9608-0218e0b91c02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e6a65-06cd-7d67-9608-0218e0b91c02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e6a65-06cd-7d92-996a-e78c76c34c86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e6a65-06cd-7d92-996a-e78c76c34c86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LettersOfCreditSublimit_019e6a65-06cd-7853-bca2-ada2d5a12625" xlink:href="cwst-20260630.xsd#cwst_LettersOfCreditSublimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_LettersOfCreditSublimit_019e6a65-06cd-7853-bca2-ada2d5a12625" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_019e6a65-06cd-71a5-b189-45e73423e80b" xlink:href="cwst-20260630.xsd#cwst_LineOfCreditFacilityAdditionalBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_019e6a65-06cd-71a5-b189-45e73423e80b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019e6a65-06cd-7f15-9d28-ae4b56f2f58b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_DebtInstrumentTerm_019e6a65-06cd-7f15-9d28-ae4b56f2f58b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityCommitmentFeeBasisSpread_019e6a65-06cd-76c9-91ed-06d1b1dd4334" xlink:href="cwst-20260630.xsd#cwst_LineOfCreditFacilityCommitmentFeeBasisSpread"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_LineOfCreditFacilityCommitmentFeeBasisSpread_019e6a65-06cd-76c9-91ed-06d1b1dd4334" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment_019e6a65-06cd-7614-b93a-c4c68e80e162" xlink:href="cwst-20260630.xsd#cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment_019e6a65-06cd-7614-b93a-c4c68e80e162" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentFloorInterestRate_019e6a65-06cd-79a1-8244-ede58d606cad" xlink:href="cwst-20260630.xsd#cwst_DebtInstrumentFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_DebtInstrumentFloorInterestRate_019e6a65-06cd-79a1-8244-ede58d606cad" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_019e6a65-06cd-7823-a460-d4d36fe0ad95" xlink:href="cwst-20260630.xsd#cwst_LineOfCreditFacilityFrontingFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_019e6a65-06cd-7823-a460-d4d36fe0ad95" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_019e6a65-06cd-7437-9da5-c60ee5217885" xlink:href="cwst-20260630.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_019e6a65-06cd-7437-9da5-c60ee5217885" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_019e6a65-06cd-7bd2-b80d-09ec129a44e3" xlink:href="cwst-20260630.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_019e6a65-06cd-7bd2-b80d-09ec129a44e3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019e6a65-06cd-737b-8a49-a0295a53af62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019e6a65-06cd-737b-8a49-a0295a53af62" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019e6a65-06cd-7c51-b2c8-dcc99e77a067" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019e6a65-06cd-7c51-b2c8-dcc99e77a067" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019f5d1d-a702-780a-809d-ad6cffe345a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019e6a65-06cd-7f1f-8105-5c6cb72a4000" xlink:to="loc_us-gaap_LineOfCredit_019f5d1d-a702-780a-809d-ad6cffe345a4" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofVariableRateMarginsDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTScheduleofVariableRateMarginsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofVariableRateMarginsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7bd7-bba8-511dcc5562ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7bd7-bba8-511dcc5562ec" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-74bd-94d0-c65a16c0b16b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-74bd-94d0-c65a16c0b16b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-74bd-94d0-c65a16c0b16b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-74bd-94d0-c65a16c0b16b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-74bd-94d0-c65a16c0b16b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-737b-8c83-9bd595a731a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-74bd-94d0-c65a16c0b16b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-737b-8c83-9bd595a731a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_019e6a65-06cd-780e-aacc-3897abb0f071" xlink:href="cwst-20260630.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-737b-8c83-9bd595a731a1" xlink:to="loc_cwst_CreditFacilityMember_019e6a65-06cd-780e-aacc-3897abb0f071" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6a65-06cd-7af1-9b33-0d0a38c38866" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:to="loc_srt_RangeAxis_019e6a65-06cd-7af1-9b33-0d0a38c38866" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-7af1-9b33-0d0a38c38866_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7af1-9b33-0d0a38c38866" xlink:to="loc_srt_RangeMember_019e6a65-06cd-7af1-9b33-0d0a38c38866_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-718e-b175-46c4ff605090" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7af1-9b33-0d0a38c38866" xlink:to="loc_srt_RangeMember_019e6a65-06cd-718e-b175-46c4ff605090" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6a65-06cd-79a5-8901-8878c14b0e91" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-718e-b175-46c4ff605090" xlink:to="loc_srt_MinimumMember_019e6a65-06cd-79a5-8901-8878c14b0e91" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6a65-06cd-7160-9d50-7aa6869550ae" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-718e-b175-46c4ff605090" xlink:to="loc_srt_MaximumMember_019e6a65-06cd-7160-9d50-7aa6869550ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-75c3-b80a-e26686b5ab46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-75c3-b80a-e26686b5ab46" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-75c3-b80a-e26686b5ab46_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-75c3-b80a-e26686b5ab46" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-75c3-b80a-e26686b5ab46_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7a58-b9ed-ea17684f4a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-75c3-b80a-e26686b5ab46" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7a58-b9ed-ea17684f4a6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-7f3a-8eb2-4535ac992b3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7a58-b9ed-ea17684f4a6b" xlink:to="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-7f3a-8eb2-4535ac992b3e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019e6a65-06cd-7550-ac5a-4797625ba0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-73a3-a176-303c9a973ca9" xlink:to="loc_us-gaap_VariableRateAxis_019e6a65-06cd-7550-ac5a-4797625ba0d5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e6a65-06cd-7550-ac5a-4797625ba0d5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019e6a65-06cd-7550-ac5a-4797625ba0d5" xlink:to="loc_us-gaap_VariableRateDomain_019e6a65-06cd-7550-ac5a-4797625ba0d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e6a65-06cd-79af-ae99-92d1e5f8782b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019e6a65-06cd-7550-ac5a-4797625ba0d5" xlink:to="loc_us-gaap_VariableRateDomain_019e6a65-06cd-79af-ae99-92d1e5f8782b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e6a65-06cd-7ea6-bb7a-fa3b900f766c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e6a65-06cd-79af-ae99-92d1e5f8782b" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e6a65-06cd-7ea6-bb7a-fa3b900f766c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019e6a65-06cd-77ad-a87e-861086cd98d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e6a65-06cd-79af-ae99-92d1e5f8782b" xlink:to="loc_us-gaap_BaseRateMember_019e6a65-06cd-77ad-a87e-861086cd98d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentVariableRateAdjustment_019e6a65-06cd-7417-bda0-5cc31823feba" xlink:href="cwst-20260630.xsd#cwst_DebtInstrumentVariableRateAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7bd7-bba8-511dcc5562ec" xlink:to="loc_cwst_DebtInstrumentVariableRateAdjustment_019e6a65-06cd-7417-bda0-5cc31823feba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6a65-06cd-7bc6-b5e5-241b593ba09d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7bd7-bba8-511dcc5562ec" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6a65-06cd-7bc6-b5e5-241b593ba09d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeTaxExemptFinancingsDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTNarrativeTaxExemptFinancingsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeTaxExemptFinancingsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7866-bb80-ba85ce55875f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-79e9-8ea3-7ab173ff14b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7866-bb80-ba85ce55875f" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-79e9-8ea3-7ab173ff14b8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-736e-8dd4-61cf547c8f22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-79e9-8ea3-7ab173ff14b8" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-736e-8dd4-61cf547c8f22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-736e-8dd4-61cf547c8f22_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-736e-8dd4-61cf547c8f22" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-736e-8dd4-61cf547c8f22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-77f8-8d4a-ca760de12dc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-736e-8dd4-61cf547c8f22" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-77f8-8d4a-ca760de12dc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7b10-8f8b-ea1c77778efb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-77f8-8d4a-ca760de12dc6" xlink:to="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7b10-8f8b-ea1c77778efb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-7303-8141-df458712e430" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-79e9-8ea3-7ab173ff14b8" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-7303-8141-df458712e430" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7303-8141-df458712e430_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-7303-8141-df458712e430" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7303-8141-df458712e430_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7095-bc49-d8c529ebfefa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6a65-06cd-7303-8141-df458712e430" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7095-bc49-d8c529ebfefa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019f5d1e-ec42-7ce5-b2a0-5e0cfaf0d372" xlink:href="cwst-20260630.xsd#cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7095-bc49-d8c529ebfefa" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019f5d1e-ec42-7ce5-b2a0-5e0cfaf0d372" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_019e6a65-06cd-7a24-906a-7cdc0328b6bb" xlink:href="cwst-20260630.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6a65-06cd-7095-bc49-d8c529ebfefa" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_019e6a65-06cd-7a24-906a-7cdc0328b6bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e6a65-06cd-7f54-8e2c-9acab3658fd6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7866-bb80-ba85ce55875f" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e6a65-06cd-7f54-8e2c-9acab3658fd6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e6a65-06cd-7361-8f70-846e9e8261dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7866-bb80-ba85ce55875f" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e6a65-06cd-7361-8f70-846e9e8261dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e6a65-06cd-7dfe-8fb3-d24f3dd0781a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7866-bb80-ba85ce55875f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e6a65-06cd-7dfe-8fb3-d24f3dd0781a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTNarrativeCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeCashFlowHedgesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e6a65-06cd-7fb8-a4cd-432579cf1c58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e6a65-06cd-7125-b0f1-82aaab4d1ce2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e6a65-06cd-7fb8-a4cd-432579cf1c58" xlink:to="loc_us-gaap_DerivativeTable_019e6a65-06cd-7125-b0f1-82aaab4d1ce2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7dbc-8b1a-50e1db707326" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6a65-06cd-7125-b0f1-82aaab4d1ce2" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7dbc-8b1a-50e1db707326" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7dbc-8b1a-50e1db707326_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7dbc-8b1a-50e1db707326" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7dbc-8b1a-50e1db707326_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7095-8952-f8f006be4e72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7dbc-8b1a-50e1db707326" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7095-8952-f8f006be4e72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_019e6a65-06cd-7d7a-a953-8252393354ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7095-8952-f8f006be4e72" xlink:to="loc_us-gaap_InterestRateContractMember_019e6a65-06cd-7d7a-a953-8252393354ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e6a65-06cd-7ef4-af2f-ae113d2f31c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6a65-06cd-7125-b0f1-82aaab4d1ce2" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e6a65-06cd-7ef4-af2f-ae113d2f31c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019e6a65-06cd-7ef4-af2f-ae113d2f31c1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e6a65-06cd-7ef4-af2f-ae113d2f31c1" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019e6a65-06cd-7ef4-af2f-ae113d2f31c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019e6a65-06cd-7616-93d5-709684205001" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e6a65-06cd-7ef4-af2f-ae113d2f31c1" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019e6a65-06cd-7616-93d5-709684205001" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_019e6a65-06cd-7457-9dfb-2dfda5a8af51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashFlowHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_019e6a65-06cd-7616-93d5-709684205001" xlink:to="loc_us-gaap_CashFlowHedgingMember_019e6a65-06cd-7457-9dfb-2dfda5a8af51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019e6a65-06cd-7be6-9b47-c821962f8980" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6a65-06cd-7fb8-a4cd-432579cf1c58" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019e6a65-06cd-7be6-9b47-c821962f8980" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFloorInterestRate_019e6a65-06cd-7aeb-bfc5-9cdefb3eabee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6a65-06cd-7fb8-a4cd-432579cf1c58" xlink:to="loc_us-gaap_DerivativeFloorInterestRate_019e6a65-06cd-7aeb-bfc5-9cdefb3eabee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAverageFixedInterestRate_019e6a65-06cd-759a-8788-e13a5419cc87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAverageFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6a65-06cd-7fb8-a4cd-432579cf1c58" xlink:to="loc_us-gaap_DerivativeAverageFixedInterestRate_019e6a65-06cd-759a-8788-e13a5419cc87" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTScheduleofCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019e6a65-06cd-7c2f-8fb2-47a648a056a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfFinancialPositionLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019e6a65-06cd-7c2f-8fb2-47a648a056a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-7006-b851-812b45f39d49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-7006-b851-812b45f39d49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e6a65-06cd-75b9-ac24-77aab547b843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e6a65-06cd-75b9-ac24-77aab547b843" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7011-9579-509d1acd8ba6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7011-9579-509d1acd8ba6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7c47-af1c-9cb78fa4e8cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7c47-af1c-9cb78fa4e8cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019e6a65-06cd-745f-89af-42d5dddd7565" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019e6a65-06cd-745f-89af-42d5dddd7565" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019e6a65-06cd-7a02-be2b-a27105f76f77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019e6a65-06cd-7a02-be2b-a27105f76f77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociBeforeTaxAttributableToParent_019e6a65-06cd-7f3c-b224-585d98db5141" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AociBeforeTaxAttributableToParent_019e6a65-06cd-7f3c-b224-585d98db5141" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociTaxAttributableToParent_019e6a65-06cd-77e9-8ac6-9f4842b6ee5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AociTaxAttributableToParent_019e6a65-06cd-77e9-8ac6-9f4842b6ee5f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-06cd-7022-a32a-362597f19449" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-06cd-7022-a32a-362597f19449" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails_1" xlink:type="simple" xlink:href="cwst-20260630.xsd#DEBTScheduleofCashFlowHedgesDetails_1"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-7006-b851-812b45f39d49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-7006-b851-812b45f39d49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6a65-06cd-7006-b851-812b45f39d49" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e6a65-06cd-75b9-ac24-77aab547b843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e6a65-06cd-797c-adc1-b473634e8021" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e6a65-06cd-75b9-ac24-77aab547b843" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6a65-06cd-7743-8d44-9af334f333aa" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7011-9579-509d1acd8ba6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7011-9579-509d1acd8ba6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-7011-9579-509d1acd8ba6" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7c47-af1c-9cb78fa4e8cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7754-b351-36a2dc027cd8" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7c47-af1c-9cb78fa4e8cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019e6a65-06cd-745f-89af-42d5dddd7565" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019e6a65-06cd-745f-89af-42d5dddd7565" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019e6a65-06cd-7a02-be2b-a27105f76f77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019e6a65-06cd-7a02-be2b-a27105f76f77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociBeforeTaxAttributableToParent_019e6a65-06cd-7f3c-b224-585d98db5141" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AociBeforeTaxAttributableToParent_019e6a65-06cd-7f3c-b224-585d98db5141" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociTaxAttributableToParent_019e6a65-06cd-77e9-8ac6-9f4842b6ee5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AociTaxAttributableToParent_019e6a65-06cd-77e9-8ac6-9f4842b6ee5f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-06cd-7022-a32a-362597f19449" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6a65-06cd-7dcd-a973-e66679cac748" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e6a65-06cd-7022-a32a-362597f19449" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#COMMITMENTSANDCONTINGENCIESNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019e6a65-06cd-7d8f-95ac-d47135338bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:to="loc_us-gaap_LossContingenciesTable_019e6a65-06cd-7d8f-95ac-d47135338bb2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019e6a65-06cd-7966-913e-454bb56ffb89" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6a65-06cd-7d8f-95ac-d47135338bb2" xlink:to="loc_srt_LitigationCaseAxis_019e6a65-06cd-7966-913e-454bb56ffb89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019e6a65-06cd-7966-913e-454bb56ffb89_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019e6a65-06cd-7966-913e-454bb56ffb89" xlink:to="loc_srt_LitigationCaseTypeDomain_019e6a65-06cd-7966-913e-454bb56ffb89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019e6a65-06cd-7fd7-a1dd-8816aeaa3286" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019e6a65-06cd-7966-913e-454bb56ffb89" xlink:to="loc_srt_LitigationCaseTypeDomain_019e6a65-06cd-7fd7-a1dd-8816aeaa3286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_019e6a65-06cd-7041-aa1e-93f530f9bf8c" xlink:href="cwst-20260630.xsd#cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019e6a65-06cd-7fd7-a1dd-8816aeaa3286" xlink:to="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_019e6a65-06cd-7041-aa1e-93f530f9bf8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019e6a65-06cd-70f9-aa35-dce5d70d4bf3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019e6a65-06cd-70f9-aa35-dce5d70d4bf3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019e6a65-06cd-7df2-877c-d4d9cc56a97c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019e6a65-06cd-7df2-877c-d4d9cc56a97c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCosts_019e6a65-06cd-75e5-b5a2-4788d584963b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:to="loc_us-gaap_DeferredCosts_019e6a65-06cd-75e5-b5a2-4788d584963b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_019e6a65-06cd-71b4-9063-f073a0872087" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6a65-06cd-743c-91ee-31ea39f6481f" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_019e6a65-06cd-71b4-9063-f073a0872087" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#STOCKHOLDERSEQUITYNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LimitedPartnersCapitalAccountLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7689-807f-2929d8023d1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7689-807f-2929d8023d1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7689-807f-2929d8023d1c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7689-807f-2929d8023d1c" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7689-807f-2929d8023d1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-74e4-9156-49ee0bd3c5ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7689-807f-2929d8023d1c" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-74e4-9156-49ee0bd3c5ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-06cd-712d-9661-79c675cc7bd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-74e4-9156-49ee0bd3c5ac" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-06cd-712d-9661-79c675cc7bd8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e6a65-06cd-779a-9f3b-0fec05eaa919" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:to="loc_us-gaap_PlanNameAxis_019e6a65-06cd-779a-9f3b-0fec05eaa919" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e6a65-06cd-779a-9f3b-0fec05eaa919_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e6a65-06cd-779a-9f3b-0fec05eaa919" xlink:to="loc_us-gaap_PlanNameDomain_019e6a65-06cd-779a-9f3b-0fec05eaa919_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e6a65-06cd-779a-9f3b-0fec05eaa919" xlink:to="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_Amended2016PlanMember_019e6a65-06cd-72bf-9814-278dc10d2e59" xlink:href="cwst-20260630.xsd#cwst_Amended2016PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:to="loc_cwst_Amended2016PlanMember_019e6a65-06cd-72bf-9814-278dc10d2e59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2016IncentivePlanMember_019e6a65-06cd-7c06-97b4-aa98d144df61" xlink:href="cwst-20260630.xsd#cwst_A2016IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:to="loc_cwst_A2016IncentivePlanMember_019e6a65-06cd-7c06-97b4-aa98d144df61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2006IncentivePlanMember_019e6a65-06cd-75b1-a987-345aa9fbc2a4" xlink:href="cwst-20260630.xsd#cwst_A2006IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:to="loc_cwst_A2006IncentivePlanMember_019e6a65-06cd-75b1-a987-345aa9fbc2a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_019e6a65-06cd-7cb9-857e-9c7bf82dc402" xlink:href="cwst-20260630.xsd#cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e6a65-06cd-7acf-a63f-e31edfd990f1" xlink:to="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_019e6a65-06cd-7cb9-857e-9c7bf82dc402" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6a65-06cd-7be7-aded-69d8c6f1f87f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:to="loc_srt_RangeAxis_019e6a65-06cd-7be7-aded-69d8c6f1f87f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-7be7-aded-69d8c6f1f87f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7be7-aded-69d8c6f1f87f" xlink:to="loc_srt_RangeMember_019e6a65-06cd-7be7-aded-69d8c6f1f87f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6a65-06cd-7275-b233-c7f6ff8cd962" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6a65-06cd-7be7-aded-69d8c6f1f87f" xlink:to="loc_srt_RangeMember_019e6a65-06cd-7275-b233-c7f6ff8cd962" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6a65-06cd-747d-a2b9-4b90d63e38b6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-7275-b233-c7f6ff8cd962" xlink:to="loc_srt_MinimumMember_019e6a65-06cd-747d-a2b9-4b90d63e38b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6a65-06cd-74a9-9536-45aab49d166d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-7275-b233-c7f6ff8cd962" xlink:to="loc_srt_MaximumMember_019e6a65-06cd-74a9-9536-45aab49d166d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019e6a65-06cd-7e33-8b8d-a52880d6bb4f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6a65-06cd-7275-b233-c7f6ff8cd962" xlink:to="loc_srt_WeightedAverageMember_019e6a65-06cd-7e33-8b8d-a52880d6bb4f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-79f2-b814-fbefd0a6450b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:to="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-79f2-b814-fbefd0a6450b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-79f2-b814-fbefd0a6450b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-79f2-b814-fbefd0a6450b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-79f2-b814-fbefd0a6450b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-79f2-b814-fbefd0a6450b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e6a65-06cd-7928-907e-379b5d3eabce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e6a65-06cd-7928-907e-379b5d3eabce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockAwardsMember_019e6a65-06cd-78a1-b48e-97080b8cde0d" xlink:href="cwst-20260630.xsd#cwst_RestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_cwst_RestrictedStockAwardsMember_019e6a65-06cd-78a1-b48e-97080b8cde0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6a65-06cd-7554-a783-edbe3cd5b438" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6a65-06cd-7554-a783-edbe3cd5b438" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019e6a65-06cd-7ee6-95ea-024eeb887f2d" xlink:href="cwst-20260630.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019e6a65-06cd-7ee6-95ea-024eeb887f2d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_019e6a65-06cd-7582-b6fc-eb03aafb162d" xlink:href="cwst-20260630.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_cwst_PerformanceStockUnitMember_019e6a65-06cd-7582-b6fc-eb03aafb162d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MarketbasedPerformanceStockUnitsMember_019e6a65-06cd-78c9-99c7-a5c197bb88d1" xlink:href="cwst-20260630.xsd#cwst_MarketbasedPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-788a-b451-0451a48cddb5" xlink:to="loc_cwst_MarketbasedPerformanceStockUnitsMember_019e6a65-06cd-78c9-99c7-a5c197bb88d1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019e6a65-06cd-7f85-875a-c9d92596f192" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019e6a65-06cd-7e89-a99d-182c5c9a97aa" xlink:to="loc_srt_TitleOfIndividualAxis_019e6a65-06cd-7f85-875a-c9d92596f192" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e6a65-06cd-7f85-875a-c9d92596f192_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019e6a65-06cd-7f85-875a-c9d92596f192" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e6a65-06cd-7f85-875a-c9d92596f192_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e6a65-06cd-79de-b64c-c35419b97c78" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019e6a65-06cd-7f85-875a-c9d92596f192" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e6a65-06cd-79de-b64c-c35419b97c78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NonEmployeeDirectorMember_019e6a65-06cd-7394-a0d0-5e2d3f69992e" xlink:href="cwst-20260630.xsd#cwst_NonEmployeeDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e6a65-06cd-79de-b64c-c35419b97c78" xlink:to="loc_cwst_NonEmployeeDirectorMember_019e6a65-06cd-7394-a0d0-5e2d3f69992e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019e6a65-06cd-719c-a0b7-ab254225458a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019e6a65-06cd-719c-a0b7-ab254225458a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e6a65-06cd-745b-98e0-f698272621ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e6a65-06cd-745b-98e0-f698272621ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e6a65-06cd-73f8-9ea2-3193e7059034" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e6a65-06cd-73f8-9ea2-3193e7059034" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e6a65-06cd-7e49-a97b-05aaa15f9528" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e6a65-06cd-7e49-a97b-05aaa15f9528" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e6a65-06cd-7d33-9aef-0914157dc74f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e6a65-06cd-7d33-9aef-0914157dc74f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e6a65-06cd-7f40-a07d-56ddca96a43d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e6a65-06cd-7f40-a07d-56ddca96a43d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019e6a65-06cd-74fc-b427-03f2452a0340" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019e6a65-06cd-74fc-b427-03f2452a0340" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e6a65-06cd-739b-85a9-a05677c60843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e6a65-06cd-739b-85a9-a05677c60843" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019e6a65-06cd-7ee7-863a-0dae2ce89eaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019e6a65-06cd-7ee7-863a-0dae2ce89eaa" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019e6a65-06cd-737d-bb46-86293f4526ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019e6a65-06cd-737d-bb46-86293f4526ac" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019e6a65-06cd-75fa-803b-a068e4f1c72a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019e6a65-06cd-75fa-803b-a068e4f1c72a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e6a65-06cd-7282-9b57-8e2103287b82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e6a65-06cd-7282-9b57-8e2103287b82" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-74d7-8d06-db1398a26a2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-74d7-8d06-db1398a26a2c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e6a65-06cd-72b6-a983-bb0c534157c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e6a65-06cd-72b6-a983-bb0c534157c4" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e6a65-06cd-79c7-bb52-cbd300d3d5c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019e6a65-06cd-7ac0-af4f-46aa5c690a6d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e6a65-06cd-79c7-bb52-cbd300d3d5c9" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6a65-06cd-7bae-b783-3fa8450d12ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6a65-06cd-7bae-b783-3fa8450d12ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019e6a65-06cd-7a1d-9bb3-1358e1801a31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6a65-06cd-7bae-b783-3fa8450d12ff" xlink:to="loc_us-gaap_VestingAxis_019e6a65-06cd-7a1d-9bb3-1358e1801a31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019e6a65-06cd-7a1d-9bb3-1358e1801a31_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019e6a65-06cd-7a1d-9bb3-1358e1801a31" xlink:to="loc_us-gaap_VestingDomain_019e6a65-06cd-7a1d-9bb3-1358e1801a31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019e6a65-06cd-77c0-997e-02da55c1f895" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019e6a65-06cd-7a1d-9bb3-1358e1801a31" xlink:to="loc_us-gaap_VestingDomain_019e6a65-06cd-77c0-997e-02da55c1f895" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019e6a65-06cd-7d0c-9f13-05ecbe156f49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019e6a65-06cd-77c0-997e-02da55c1f895" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019e6a65-06cd-7d0c-9f13-05ecbe156f49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-7a3b-aac5-4287f13d9a2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6a65-06cd-7bae-b783-3fa8450d12ff" xlink:to="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-7a3b-aac5-4287f13d9a2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-7a3b-aac5-4287f13d9a2d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-7a3b-aac5-4287f13d9a2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-7a3b-aac5-4287f13d9a2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-7d8a-8ef4-6d88730a975b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e6a65-06cd-7a3b-aac5-4287f13d9a2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-7d8a-8ef4-6d88730a975b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019e6a65-06cd-729d-b7dd-c65f46264330" xlink:href="cwst-20260630.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6a65-06cd-7d8a-8ef4-6d88730a975b" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019e6a65-06cd-729d-b7dd-c65f46264330" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_019e6a65-06cd-77a1-844c-81fa49896126" xlink:href="cwst-20260630.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019e6a65-06cd-729d-b7dd-c65f46264330" xlink:to="loc_cwst_PerformanceStockUnitMember_019e6a65-06cd-77a1-844c-81fa49896126" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7f22-90dd-c67f9f0b2271" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6a65-06cd-7bae-b783-3fa8450d12ff" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7f22-90dd-c67f9f0b2271" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7f22-90dd-c67f9f0b2271_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7f22-90dd-c67f9f0b2271" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7f22-90dd-c67f9f0b2271_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-73d5-a9da-4685dd67d12e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7f22-90dd-c67f9f0b2271" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-73d5-a9da-4685dd67d12e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-06cd-7fd5-8897-6fe7dc37678d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-73d5-a9da-4685dd67d12e" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-06cd-7fd5-8897-6fe7dc37678d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6a65-06cd-7f10-9cd8-8e9ea0edb287" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6a65-06cd-7f10-9cd8-8e9ea0edb287" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e6a65-06cd-7825-89a7-92a363acab15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e6a65-06cd-7825-89a7-92a363acab15" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e6a65-06cd-752f-8475-4a84fb4d7568" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e6a65-06cd-752f-8475-4a84fb4d7568" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e6a65-06cd-793e-90be-a2cec112ad84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6a65-06cd-7a34-88e2-00c7e81c9042" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e6a65-06cd-793e-90be-a2cec112ad84" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6a65-06cd-7ee9-937b-54a9e703e2eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6a65-06cd-797e-b593-38371385fe64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6a65-06cd-797e-b593-38371385fe64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-7ff0-8f03-88e96c8d0121" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-7ff0-8f03-88e96c8d0121" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-71e1-829b-7915d865dabf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e6a65-06cd-71e1-829b-7915d865dabf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e6a65-06cd-7b08-aa60-6fd903dd687c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6a65-06cd-7219-a606-9e1565f3319e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e6a65-06cd-7b08-aa60-6fd903dd687c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6a65-06cd-7a80-a654-37a6feefa4c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019e6a65-06cd-78b5-a09c-7d886ae35736" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019e6a65-06cd-78b5-a09c-7d886ae35736" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019e6a65-06cd-78e0-94e6-3a290eac314c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019e6a65-06cd-78b5-a09c-7d886ae35736" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019e6a65-06cd-78e0-94e6-3a290eac314c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019e6a65-06cd-73f7-ba96-cc204bf508eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019e6a65-06cd-78b5-a09c-7d886ae35736" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019e6a65-06cd-73f7-ba96-cc204bf508eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019e6a65-06cd-7226-9015-9f94de0da175" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019e6a65-06cd-7226-9015-9f94de0da175" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_019e6a65-06cd-74e4-82ce-70eda16097fc" xlink:href="cwst-20260630.xsd#cwst_AdditionalSharesIssuedMaximumAttainmentLevel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6a65-06cd-77fa-84a3-55da4a14fcca" xlink:to="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_019e6a65-06cd-74e4-82ce-70eda16097fc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e6a65-06cd-77ad-a244-e606b11594b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e6a65-06cd-755e-ac7a-a98d2806949b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e6a65-06cd-77ad-a244-e606b11594b6" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e6a65-06cd-755e-ac7a-a98d2806949b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7f1b-b05e-6d44a2978fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e6a65-06cd-755e-ac7a-a98d2806949b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7f1b-b05e-6d44a2978fd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-7f1b-b05e-6d44a2978fd5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7f1b-b05e-6d44a2978fd5" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-7f1b-b05e-6d44a2978fd5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-77be-b249-6c68aecd269b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7f1b-b05e-6d44a2978fd5" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-77be-b249-6c68aecd269b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e6a65-06cd-70a3-aec7-81b527c49c04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-77be-b249-6c68aecd269b" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e6a65-06cd-70a3-aec7-81b527c49c04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e6a65-06cd-77ad-a244-e606b11594b6" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e6a65-06cd-7729-9ea2-348119066655" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:to="loc_us-gaap_StockholdersEquity_019e6a65-06cd-7729-9ea2-348119066655" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019e6a65-06cd-7c69-a7d8-9711386b9299" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:to="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019e6a65-06cd-7c69-a7d8-9711386b9299" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019e6a65-06cd-746e-b36f-d2bece08ce7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019e6a65-06cd-746e-b36f-d2bece08ce7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019e6a65-06cd-7883-9a85-44635b0567f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019e6a65-06cd-7883-9a85-44635b0567f8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e6a65-06cd-77c0-b262-063bbc19b6d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019e6a65-06cd-7085-be6a-cd3d6b2ce16a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e6a65-06cd-77c0-b262-063bbc19b6d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e6a65-06cd-7987-a00f-f108eadbccdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019e6a65-06cd-735e-b5f1-980c1e2591a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019e6a65-06cd-735e-b5f1-980c1e2591a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019e6a65-06cd-735e-b5f1-980c1e2591a5" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-75b7-82b9-8f38cd9d91ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6a65-06cd-7b8a-95d0-8ce7bb7b8cb1" xlink:to="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-75b7-82b9-8f38cd9d91ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e6a65-06cd-76b8-ab4a-eeca4990e3d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6a65-06cd-75b7-82b9-8f38cd9d91ea" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e6a65-06cd-76b8-ab4a-eeca4990e3d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e6a65-06cd-72ea-bdfa-dc70f7a5f879" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019e6a65-06cd-735e-b5f1-980c1e2591a5" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e6a65-06cd-72ea-bdfa-dc70f7a5f879" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e6a65-06cd-72ea-bdfa-dc70f7a5f879_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e6a65-06cd-72ea-bdfa-dc70f7a5f879" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e6a65-06cd-72ea-bdfa-dc70f7a5f879_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e6a65-06cd-7186-bd93-d5b2b09d7306" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e6a65-06cd-72ea-bdfa-dc70f7a5f879" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e6a65-06cd-7186-bd93-d5b2b09d7306" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e6a65-06cd-7b76-b665-c96d1988d85b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e6a65-06cd-7186-bd93-d5b2b09d7306" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e6a65-06cd-7b76-b665-c96d1988d85b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019e6a65-06cd-789a-8d2c-30d34391442f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019e6a65-06cd-789a-8d2c-30d34391442f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e6a65-06cd-749e-8d48-deac1d10e3bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e6a65-06cd-749e-8d48-deac1d10e3bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e6a65-06cd-735f-b078-12a1076efa82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e6a65-06cd-735f-b078-12a1076efa82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e6a65-06cd-794b-8d35-7ecf25d5508e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019e6a65-06cd-75aa-8e09-a906f4ef5fd5" xlink:to="loc_us-gaap_NetIncomeLoss_019e6a65-06cd-794b-8d35-7ecf25d5508e" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/EARNINGSLOSSPERSHAREDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#EARNINGSLOSSPERSHAREDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/EARNINGSLOSSPERSHAREDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e6a65-06cd-73f1-812c-de846a433876" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e6a65-06cd-73f1-812c-de846a433876" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7461-9f6a-152f20dd471e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e6a65-06cd-73f1-812c-de846a433876" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7461-9f6a-152f20dd471e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7461-9f6a-152f20dd471e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7461-9f6a-152f20dd471e" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7461-9f6a-152f20dd471e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7963-ac15-ad23e57841d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e6a65-06cd-7461-9f6a-152f20dd471e" xlink:to="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7963-ac15-ad23e57841d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e6a65-06cd-7f5f-b9e0-046b47c8b432" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7963-ac15-ad23e57841d4" xlink:to="loc_us-gaap_CommonClassAMember_019e6a65-06cd-7f5f-b9e0-046b47c8b432" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e6a65-06cd-7a9a-93b0-84167eaf5568" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e6a65-06cd-7963-ac15-ad23e57841d4" xlink:to="loc_us-gaap_CommonClassBMember_019e6a65-06cd-7a9a-93b0-84167eaf5568" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019e6a65-06cd-7637-9383-cbf0379e9236" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019e6a65-06cd-7637-9383-cbf0379e9236" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperations_019e6a65-06cd-791f-a346-0943f480676e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019e6a65-06cd-7637-9383-cbf0379e9236" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperations_019e6a65-06cd-791f-a346-0943f480676e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019e6a65-06cd-782e-a749-c5407987c793" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019e6a65-06cd-782e-a749-c5407987c793" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019fb3b0-19ed-79d1-ba9d-e116790ebd69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019e6a65-06cd-782e-a749-c5407987c793" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019fb3b0-19ed-79d1-ba9d-e116790ebd69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_019f8a23-ded0-7fe6-b37c-c84369607c78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesIssuedBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019e6a65-06cd-782e-a749-c5407987c793" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_019f8a23-ded0-7fe6-b37c-c84369607c78" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019e6a65-06cd-7ec3-a5ac-1bb591637b86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019e6a65-06cd-782e-a749-c5407987c793" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019e6a65-06cd-7ec3-a5ac-1bb591637b86" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019e6a65-06cd-74e3-a812-edb8eb270ec8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019e6a65-06cd-74e3-a812-edb8eb270ec8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019e6a65-06cd-70ef-a316-e98fa245cb93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019e6a65-06cd-74e3-a812-edb8eb270ec8" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019e6a65-06cd-70ef-a316-e98fa245cb93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019e6a65-06cd-7f60-a10d-e876c65cf21b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019e6a65-06cd-7f60-a10d-e876c65cf21b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e6a65-06cd-72de-84d2-6571f65ede30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e6a65-06cd-7d31-9d79-33d03dcfc481" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e6a65-06cd-72de-84d2-6571f65ede30" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e6a65-06cd-7d31-9561-8ad2ceb48386" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e6a65-06cd-7b54-a963-26a4f64b94ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e6a65-06cd-7d31-9561-8ad2ceb48386" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e6a65-06cd-7b54-a963-26a4f64b94ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e6a65-06cd-74b0-bb46-918f1ff302da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e6a65-06cd-7b54-a963-26a4f64b94ab" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e6a65-06cd-74b0-bb46-918f1ff302da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-74b0-bb46-918f1ff302da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e6a65-06cd-74b0-bb46-918f1ff302da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-74b0-bb46-918f1ff302da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-76f8-b15b-995fa0f2d0a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e6a65-06cd-74b0-bb46-918f1ff302da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-76f8-b15b-995fa0f2d0a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e6a65-06cd-703d-a4fd-62d5f612f119" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-76f8-b15b-995fa0f2d0a4" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e6a65-06cd-703d-a4fd-62d5f612f119" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e6a65-06cd-75f0-952c-099f3ac281e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-76f8-b15b-995fa0f2d0a4" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e6a65-06cd-75f0-952c-099f3ac281e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e6a65-06cd-7341-b821-0685130fdc90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e6a65-06cd-76f8-b15b-995fa0f2d0a4" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e6a65-06cd-7341-b821-0685130fdc90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e6a65-06cd-7a8a-8127-35a710733605" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e6a65-06cd-7b54-a963-26a4f64b94ab" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e6a65-06cd-7a8a-8127-35a710733605" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e6a65-06cd-7a8a-8127-35a710733605_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e6a65-06cd-7a8a-8127-35a710733605" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e6a65-06cd-7a8a-8127-35a710733605_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e6a65-06cd-78ed-963a-fa43149f27f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e6a65-06cd-7a8a-8127-35a710733605" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e6a65-06cd-78ed-963a-fa43149f27f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_019e6a65-06cd-7b4a-80e6-31d97ef3f8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e6a65-06cd-78ed-963a-fa43149f27f6" xlink:to="loc_us-gaap_LandfillMember_019e6a65-06cd-7b4a-80e6-31d97ef3f8ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-763f-ac61-8d5964cc251e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e6a65-06cd-7b54-a963-26a4f64b94ab" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-763f-ac61-8d5964cc251e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-763f-ac61-8d5964cc251e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-763f-ac61-8d5964cc251e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-763f-ac61-8d5964cc251e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7507-9f0a-ab30b606faef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6a65-06cd-763f-ac61-8d5964cc251e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7507-9f0a-ab30b606faef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7834-9b4d-2a4a017da286" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6a65-06cd-7507-9f0a-ab30b606faef" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6a65-06cd-7834-9b4d-2a4a017da286" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019e6a65-06cd-73f9-b9ae-b5947cf00a52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e6a65-06cd-7d31-9561-8ad2ceb48386" xlink:to="loc_us-gaap_AssetsAbstract_019e6a65-06cd-73f9-b9ae-b5947cf00a52" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019e6a65-06cd-70c5-a8cb-06e8aa46d5ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-06cd-73f9-b9ae-b5947cf00a52" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019e6a65-06cd-70c5-a8cb-06e8aa46d5ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019e6a65-06cd-71bc-b223-5ecd9ba26088" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-06cd-73f9-b9ae-b5947cf00a52" xlink:to="loc_us-gaap_DerivativeAssets_019e6a65-06cd-71bc-b223-5ecd9ba26088" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019e6a65-06cd-7786-9bbc-fde929207fe4" xlink:href="cwst-20260630.xsd#cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssets_019e6a65-06cd-71bc-b223-5ecd9ba26088" xlink:to="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019e6a65-06cd-7786-9bbc-fde929207fe4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019e6a65-06cd-73a5-bf84-94e2e67bcd9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e6a65-06cd-73f9-b9ae-b5947cf00a52" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019e6a65-06cd-73a5-bf84-94e2e67bcd9f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019e6a65-06cd-7064-a83e-4dd5ce841c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e6a65-06cd-7d31-9561-8ad2ceb48386" xlink:to="loc_us-gaap_LiabilitiesAbstract_019e6a65-06cd-7064-a83e-4dd5ce841c9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_019e6a65-06cd-790d-afdf-10d449aebc17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateDerivativeLiabilitiesAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019e6a65-06cd-7064-a83e-4dd5ce841c9d" xlink:to="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_019e6a65-06cd-790d-afdf-10d449aebc17" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c08-9f2e-c7bb3b96a18e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7cec-8f8f-0e9aa51c4e40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c08-9f2e-c7bb3b96a18e" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7cec-8f8f-0e9aa51c4e40" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6a65-06cd-7ca5-80a6-ad374a1d948c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7cec-8f8f-0e9aa51c4e40" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6a65-06cd-7ca5-80a6-ad374a1d948c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7ca5-80a6-ad374a1d948c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6a65-06cd-7ca5-80a6-ad374a1d948c" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7ca5-80a6-ad374a1d948c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6a65-06cd-7e64-8b60-0b89c71e5338" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6a65-06cd-7ca5-80a6-ad374a1d948c" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6a65-06cd-7e64-8b60-0b89c71e5338" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7b0a-8dc0-83858a4705d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6a65-06cd-7e64-8b60-0b89c71e5338" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7b0a-8dc0-83858a4705d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e6a65-06cd-74f9-8f0c-abb56808a48d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7b0a-8dc0-83858a4705d2" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e6a65-06cd-74f9-8f0c-abb56808a48d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e6a65-06cd-75a3-84d6-f71fbd5291aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6a65-06cd-7b0a-8dc0-83858a4705d2" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e6a65-06cd-75a3-84d6-f71fbd5291aa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-755e-9e43-0b0166de2828" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6a65-06cd-7cec-8f8f-0e9aa51c4e40" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-755e-9e43-0b0166de2828" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-755e-9e43-0b0166de2828_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-755e-9e43-0b0166de2828" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-755e-9e43-0b0166de2828_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7671-b10c-c5d7f0ff7acf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6a65-06cd-755e-9e43-0b0166de2828" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7671-b10c-c5d7f0ff7acf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7ae6-a352-db1c8a13bf93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7671-b10c-c5d7f0ff7acf" xlink:to="loc_us-gaap_SecuredDebtMember_019e6a65-06cd-7ae6-a352-db1c8a13bf93" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-777e-90b3-d0a4f3bb277d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7671-b10c-c5d7f0ff7acf" xlink:to="loc_us-gaap_LineOfCreditMember_019e6a65-06cd-777e-90b3-d0a4f3bb277d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7140-8618-a3c9de37948b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6a65-06cd-7671-b10c-c5d7f0ff7acf" xlink:to="loc_us-gaap_UnsecuredDebtMember_019e6a65-06cd-7140-8618-a3c9de37948b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019e6a65-06cd-763b-8ada-1a173bb1dae6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6a65-06cd-7c08-9f2e-c7bb3b96a18e" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019e6a65-06cd-763b-8ada-1a173bb1dae6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#SEGMENTREPORTINGNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-751e-9bee-8ee1838c64fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7e1d-b5f0-dfd2b34e4702" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-751e-9bee-8ee1838c64fb" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7e1d-b5f0-dfd2b34e4702" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7abb-88d3-bd73c228dcfa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7e1d-b5f0-dfd2b34e4702" xlink:to="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7abb-88d3-bd73c228dcfa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7abb-88d3-bd73c228dcfa_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7abb-88d3-bd73c228dcfa" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7abb-88d3-bd73c228dcfa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7454-997a-d198375ed521" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7abb-88d3-bd73c228dcfa" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7454-997a-d198375ed521" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a37-87a4-2c2b9c761927" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7454-997a-d198375ed521" xlink:to="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a37-87a4-2c2b9c761927" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019e6a65-06cd-7ddd-9ebf-4c6c62abd291" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-751e-9bee-8ee1838c64fb" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019e6a65-06cd-7ddd-9ebf-4c6c62abd291" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019e6a65-06cd-72f1-991a-356de41f31bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-751e-9bee-8ee1838c64fb" xlink:to="loc_us-gaap_NumberOfReportableSegments_019e6a65-06cd-72f1-991a-356de41f31bc" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7b65-ac00-0544a0587484" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7b65-ac00-0544a0587484" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e6a65-06cd-7b68-92c3-b79958d72f9e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7b65-ac00-0544a0587484" xlink:to="loc_srt_ConsolidationItemsAxis_019e6a65-06cd-7b68-92c3-b79958d72f9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-7b68-92c3-b79958d72f9e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e6a65-06cd-7b68-92c3-b79958d72f9e" xlink:to="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-7b68-92c3-b79958d72f9e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-778e-b2d1-af61b9663ea5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e6a65-06cd-7b68-92c3-b79958d72f9e" xlink:to="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-778e-b2d1-af61b9663ea5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_019e6a65-06cd-7ec6-aa08-1f754d3b2e5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-778e-b2d1-af61b9663ea5" xlink:to="loc_us-gaap_IntersegmentEliminationMember_019e6a65-06cd-7ec6-aa08-1f754d3b2e5b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e6a65-06cd-7396-be07-79f6298f311c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-778e-b2d1-af61b9663ea5" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e6a65-06cd-7396-be07-79f6298f311c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019e6a65-06cd-7aa0-bc68-5ee7a2fc009e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6a65-06cd-778e-b2d1-af61b9663ea5" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019e6a65-06cd-7aa0-bc68-5ee7a2fc009e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-76f3-b1f1-ee45fce31486" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7b65-ac00-0544a0587484" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-76f3-b1f1-ee45fce31486" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-76f3-b1f1-ee45fce31486_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-76f3-b1f1-ee45fce31486" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-76f3-b1f1-ee45fce31486_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6a65-06cd-791e-a94f-cfd00d60c7d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6a65-06cd-76f3-b1f1-ee45fce31486" xlink:to="loc_us-gaap_SegmentDomain_019e6a65-06cd-791e-a94f-cfd00d60c7d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-793e-a4b5-7adf1d7941b0" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-791e-a94f-cfd00d60c7d1" xlink:to="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-793e-a4b5-7adf1d7941b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-765a-a1a8-758c925acd15" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6a65-06cd-791e-a94f-cfd00d60c7d1" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019e6a65-06cd-765a-a1a8-758c925acd15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_019e6a65-06cd-7535-9e6d-bc36e8ab687c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6a65-06cd-7b65-ac00-0544a0587484" xlink:to="loc_us-gaap_SubsegmentsAxis_019e6a65-06cd-7535-9e6d-bc36e8ab687c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7535-9e6d-bc36e8ab687c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_019e6a65-06cd-7535-9e6d-bc36e8ab687c" xlink:to="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7535-9e6d-bc36e8ab687c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7ee3-b1d9-2d4c508862ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_019e6a65-06cd-7535-9e6d-bc36e8ab687c" xlink:to="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7ee3-b1d9-2d4c508862ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019e6a65-06cd-7e36-819d-0a7b2fe081be" xlink:href="cwst-20260630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7ee3-b1d9-2d4c508862ba" xlink:to="loc_cwst_EasternRegionMember_019e6a65-06cd-7e36-819d-0a7b2fe081be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019e6a65-06cd-7c8c-bd98-9aa9e31bf48c" xlink:href="cwst-20260630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7ee3-b1d9-2d4c508862ba" xlink:to="loc_cwst_WesternRegionMember_019e6a65-06cd-7c8c-bd98-9aa9e31bf48c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7590-a7e2-a24e6afef815" xlink:href="cwst-20260630.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019e6a65-06cd-7ee3-b1d9-2d4c508862ba" xlink:to="loc_cwst_MidAtlanticRegionMember_019e6a65-06cd-7590-a7e2-a24e6afef815" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019e6a65-06cd-766a-b1e1-6c5b00cb7ad0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_Revenues_019e6a65-06cd-766a-b1e1-6c5b00cb7ad0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019e6a65-06cd-7d6e-850b-fddd19d03cf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019e6a65-06cd-7d6e-850b-fddd19d03cf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019e6a65-06cd-7f2e-bf11-7b8f580447e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019e6a65-06cd-7f2e-bf11-7b8f580447e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019e6a65-06cd-758e-b34a-fb8ac38072b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019e6a65-06cd-758e-b34a-fb8ac38072b7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019e6a65-06cd-783a-95ba-52bcfe9328ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019e6a65-06cd-783a-95ba-52bcfe9328ea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OrganicsFacilityClosureCharge_019e6a65-06cd-7778-a049-5e84e0f4bf22" xlink:href="cwst-20260630.xsd#cwst_OrganicsFacilityClosureCharge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_cwst_OrganicsFacilityClosureCharge_019e6a65-06cd-7778-a049-5e84e0f4bf22" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019e6a65-06cd-7212-ab82-3f02aef48ab2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_OperatingIncomeLoss_019e6a65-06cd-7212-ab82-3f02aef48ab2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019e6a65-06cd-7c11-bfb5-ab889da3f108" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019e6a65-06cd-7c11-bfb5-ab889da3f108" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e6a65-06cd-70db-ba02-98bb79ae88a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e6a65-06cd-70db-ba02-98bb79ae88a2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e6a65-06cd-7a85-89e5-ec02a25be8df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e6a65-06cd-7a85-89e5-ec02a25be8df" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e6a65-06cd-7102-a43e-407a1ca7075b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e6a65-06cd-7102-a43e-407a1ca7075b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e6a65-06cd-7c3c-b5bb-f17b02f0ecdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6a65-06cd-744e-bc7d-e6e258cbfbdd" xlink:to="loc_us-gaap_Assets_019e6a65-06cd-7c3c-b5bb-f17b02f0ecdb" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails" xlink:type="simple" xlink:href="cwst-20260630.xsd#SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6a65-06cd-73a5-bda4-b3cb1b7fa26e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6a65-06cd-7ab8-98db-c613abf87c61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6a65-06cd-73a5-bda4-b3cb1b7fa26e" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6a65-06cd-7ab8-98db-c613abf87c61" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7b70-9739-b5cb1818371b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6a65-06cd-7ab8-98db-c613abf87c61" xlink:to="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7b70-9739-b5cb1818371b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7b70-9739-b5cb1818371b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7b70-9739-b5cb1818371b" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7b70-9739-b5cb1818371b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7d80-9128-fb685e1ed111" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6a65-06cd-7b70-9739-b5cb1818371b" xlink:to="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7d80-9128-fb685e1ed111" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7d80-9128-fb685e1ed111" xlink:to="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsCollectionMember_019e6a65-06cd-7b83-bcac-47e836d7ed4d" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsCollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:to="loc_cwst_SolidWasteOperationsCollectionMember_019e6a65-06cd-7b83-bcac-47e836d7ed4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsDisposalMember_019e6a65-06cd-7a97-9c8e-02526332a46b" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsDisposalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:to="loc_cwst_SolidWasteOperationsDisposalMember_019e6a65-06cd-7a97-9c8e-02526332a46b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsTransportationMember_019e6a65-06cd-71e3-b6cd-24359e270421" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:to="loc_cwst_SolidWasteOperationsTransportationMember_019e6a65-06cd-71e3-b6cd-24359e270421" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_019e6a65-06cd-722d-8202-fa635196709e" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsLandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:to="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_019e6a65-06cd-722d-8202-fa635196709e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsProcessingMember_019e6a65-06cd-73f3-b076-56eec14f7a28" xlink:href="cwst-20260630.xsd#cwst_SolidWasteOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019e6a65-06cd-7a7e-9826-70be10e41911" xlink:to="loc_cwst_SolidWasteOperationsProcessingMember_019e6a65-06cd-73f3-b076-56eec14f7a28" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsMember_019e6a65-06cd-7aee-978f-21bfadd55527" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6a65-06cd-7d80-9128-fb685e1ed111" xlink:to="loc_cwst_ResourceSolutionsOperationsMember_019e6a65-06cd-7aee-978f-21bfadd55527" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsProcessingMember_019e6a65-06cd-79e8-a05e-1c3b40ff4ef7" xlink:href="cwst-20260630.xsd#cwst_ResourceSolutionsOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_019e6a65-06cd-7aee-978f-21bfadd55527" xlink:to="loc_cwst_ResourceSolutionsOperationsProcessingMember_019e6a65-06cd-79e8-a05e-1c3b40ff4ef7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_019e6a65-06cd-7665-8622-456c9ce96e52" xlink:href="cwst-20260630.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_019e6a65-06cd-7aee-978f-21bfadd55527" xlink:to="loc_cwst_NationalAccountsMember_019e6a65-06cd-7665-8622-456c9ce96e52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019e6a65-06cd-795d-a103-6b133b177404" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6a65-06cd-73a5-bda4-b3cb1b7fa26e" xlink:to="loc_us-gaap_Revenues_019e6a65-06cd-795d-a103-6b133b177404" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-roles-2026.xsd#eedm1"/>
  <link:definitionLink xlink:role="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:type="arc" order="31"/>
  </link:definitionLink>
</link:linkbase>
