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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019fb42f-387f-70ed-9481-1df5319d8c04,g:2ba89b84-adca-4392-935d-a2246c55e91f-->
<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://prod.itgcomm.com/role/Cover" xlink:type="simple" xlink:href="itg-20260630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019fb4ed-e8f1-7e9b-9759-a2401aebe6b4" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentInformationTable_019fb4ed-e8f1-7e9b-9759-a2401aebe6b4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fb4ed-e8f1-7d48-9324-cf406fca1846" 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_019fb4ed-e8f1-7e9b-9759-a2401aebe6b4" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fb4ed-e8f1-7d48-9324-cf406fca1846" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fb4ed-e8f1-7d48-9324-cf406fca1846_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_019fb4ed-e8f1-7d48-9324-cf406fca1846" xlink:to="loc_us-gaap_ClassOfStockDomain_019fb4ed-e8f1-7d48-9324-cf406fca1846_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fb4ed-e8f1-7b6e-80ad-de4d2e58483c" 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_019fb4ed-e8f1-7d48-9324-cf406fca1846" xlink:to="loc_us-gaap_ClassOfStockDomain_019fb4ed-e8f1-7b6e-80ad-de4d2e58483c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019fb4ed-e8f1-7a1e-a4a7-c81d68c1dc06" 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_019fb4ed-e8f1-7b6e-80ad-de4d2e58483c" xlink:to="loc_us-gaap_CommonClassAMember_019fb4ed-e8f1-7a1e-a4a7-c81d68c1dc06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019fb4ed-e8f1-7f12-ae74-f26df46feb9a" 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_019fb4ed-e8f1-7b6e-80ad-de4d2e58483c" xlink:to="loc_us-gaap_CommonClassBMember_019fb4ed-e8f1-7f12-ae74-f26df46feb9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019fb4ed-e8f1-76b9-acd7-8183f56932e2" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentType_019fb4ed-e8f1-76b9-acd7-8183f56932e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019fb4ed-e8f1-7845-967c-a8d679331f65" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentQuarterlyReport_019fb4ed-e8f1-7845-967c-a8d679331f65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019fb4ed-e8f1-7bec-9f57-9e122a04ebe5" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentPeriodEndDate_019fb4ed-e8f1-7bec-9f57-9e122a04ebe5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019fb4ed-e8f1-7617-9538-fa742a4f1e02" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentTransitionReport_019fb4ed-e8f1-7617-9538-fa742a4f1e02" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019fb4ed-e8f1-70bc-b83e-dc1e5fb3b76a" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityFileNumber_019fb4ed-e8f1-70bc-b83e-dc1e5fb3b76a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019fb4ed-e8f1-7951-9c1a-3b02a53a1ab9" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityRegistrantName_019fb4ed-e8f1-7951-9c1a-3b02a53a1ab9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019fb4ed-e8f1-73f7-81b4-db3f72298be0" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019fb4ed-e8f1-73f7-81b4-db3f72298be0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019fb4ed-e8f1-75d4-95a6-dbe23024eb84" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityTaxIdentificationNumber_019fb4ed-e8f1-75d4-95a6-dbe23024eb84" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019fb4ed-e8f1-7648-9763-e144862becd5" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityAddressAddressLine1_019fb4ed-e8f1-7648-9763-e144862becd5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_019fcfe8-652d-749c-b7d6-e49d0180f2fa" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityAddressAddressLine2_019fcfe8-652d-749c-b7d6-e49d0180f2fa" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019fcfe8-0959-79d4-bf58-98c433323d16" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityAddressCityOrTown_019fcfe8-0959-79d4-bf58-98c433323d16" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019fcfe8-2eb8-7c5a-965b-7e7bb5683d8e" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityAddressStateOrProvince_019fcfe8-2eb8-7c5a-965b-7e7bb5683d8e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019fb4ed-e8f1-7221-82d3-43b4dc6b4e33" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityAddressPostalZipCode_019fb4ed-e8f1-7221-82d3-43b4dc6b4e33" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019fb4ed-e8f1-73cd-8fea-7747c4e01a75" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_CityAreaCode_019fb4ed-e8f1-73cd-8fea-7747c4e01a75" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019fb4ed-e8f1-72b4-87a8-85c9ab883389" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_LocalPhoneNumber_019fb4ed-e8f1-72b4-87a8-85c9ab883389" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019fb4ed-e8f1-7582-8423-662230d8c4f6" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_Security12bTitle_019fb4ed-e8f1-7582-8423-662230d8c4f6" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019fb4ed-e8f1-7838-938e-747f57b52cb1" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_TradingSymbol_019fb4ed-e8f1-7838-938e-747f57b52cb1" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019fb4ed-e8f1-78b3-926e-39ac0437316a" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_SecurityExchangeName_019fb4ed-e8f1-78b3-926e-39ac0437316a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019fb4ed-e8f1-7153-adf6-e3b52ee90388" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityCurrentReportingStatus_019fb4ed-e8f1-7153-adf6-e3b52ee90388" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019fb4ed-e8f1-7995-9cbe-192fd13358c4" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityInteractiveDataCurrent_019fb4ed-e8f1-7995-9cbe-192fd13358c4" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019fb4ed-e8f1-740a-9a49-cb42e383aad4" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityFilerCategory_019fb4ed-e8f1-740a-9a49-cb42e383aad4" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019fb4ed-e8f1-720d-ae32-d8656bf4cf49" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntitySmallBusiness_019fb4ed-e8f1-720d-ae32-d8656bf4cf49" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019fb4ed-e8f1-784a-a209-ee26459c8ad1" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityEmergingGrowthCompany_019fb4ed-e8f1-784a-a209-ee26459c8ad1" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_019fbf74-bf05-75a7-9e02-a336eb337da3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityExTransitionPeriod_019fbf74-bf05-75a7-9e02-a336eb337da3" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019fb4ed-e8f1-713c-a8f8-087242348c5e" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityShellCompany_019fb4ed-e8f1-713c-a8f8-087242348c5e" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019fb4ed-e8f1-7df5-af5a-6ceb5f3cf1ec" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019fb4ed-e8f1-7df5-af5a-6ceb5f3cf1ec" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019fb4ed-e8f1-790e-844b-694c6a25bd85" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_CurrentFiscalYearEndDate_019fb4ed-e8f1-790e-844b-694c6a25bd85" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019fb4ed-e8f1-742c-ac9e-b86c501f32e7" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019fb4ed-e8f1-742c-ac9e-b86c501f32e7" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019fb4ed-e8f1-7cd6-9f6d-9869384ae955" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_AmendmentFlag_019fb4ed-e8f1-7cd6-9f6d-9869384ae955" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019fb4ed-e8f1-7682-97a5-7c75d285797a" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_DocumentFiscalYearFocus_019fb4ed-e8f1-7682-97a5-7c75d285797a" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019fb4ed-e8f1-7cc7-90bf-31ef2344b049" 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_019fb4ed-e8f1-7324-a5d0-c7db37846fbe" xlink:to="loc_dei_EntityCentralIndexKey_019fb4ed-e8f1-7cc7-90bf-31ef2344b049" xlink:type="arc" order="30"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/BalanceSheetsUnaudited" xlink:type="simple" xlink:href="itg-20260630.xsd#BalanceSheetsUnaudited"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/BalanceSheetsUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019fb4ed-e8f1-7e92-9ae3-29fda4b019e5" 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_019fb4ed-e8f1-713a-85ca-ed792c1414b4" 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_019fb4ed-e8f1-7e92-9ae3-29fda4b019e5" xlink:to="loc_us-gaap_StatementTable_019fb4ed-e8f1-713a-85ca-ed792c1414b4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-767a-a8b0-9407a94848aa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019fb4ed-e8f1-713a-85ca-ed792c1414b4" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-767a-a8b0-9407a94848aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-767a-a8b0-9407a94848aa_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-767a-a8b0-9407a94848aa" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-767a-a8b0-9407a94848aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-7526-b9d7-392752beb3b6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-767a-a8b0-9407a94848aa" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-7526-b9d7-392752beb3b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_019fb4ed-e8f1-79ac-a604-117e7f070f07" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-7526-b9d7-392752beb3b6" xlink:to="loc_srt_ParentCompanyMember_019fb4ed-e8f1-79ac-a604-117e7f070f07" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019fb4ed-e8f1-74d3-9c7b-8cc03ea880b6" 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_019fb4ed-e8f1-7e92-9ae3-29fda4b019e5" xlink:to="loc_us-gaap_AssetsAbstract_019fb4ed-e8f1-74d3-9c7b-8cc03ea880b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019fb4ed-e8f1-7f40-9cb3-867017ff1a14" 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_019fb4ed-e8f1-74d3-9c7b-8cc03ea880b6" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019fb4ed-e8f1-7f40-9cb3-867017ff1a14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Cash_019fb4ed-e8f1-79c4-aa58-7a0538334793" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Cash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019fb4ed-e8f1-7f40-9cb3-867017ff1a14" xlink:to="loc_us-gaap_Cash_019fb4ed-e8f1-79c4-aa58-7a0538334793" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019fb4ed-e8f1-76b0-bfc0-ef944a7e8ffc" 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_019fb4ed-e8f1-74d3-9c7b-8cc03ea880b6" xlink:to="loc_us-gaap_Assets_019fb4ed-e8f1-76b0-bfc0-ef944a7e8ffc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_019fb4ed-e8f1-70e0-b536-f883c10d8f9a" 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_StatementLineItems_019fb4ed-e8f1-7e92-9ae3-29fda4b019e5" xlink:to="loc_us-gaap_CommitmentsAndContingencies_019fb4ed-e8f1-70e0-b536-f883c10d8f9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019fb4ed-e8f1-76b2-89dd-22ca651622e4" 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_StatementLineItems_019fb4ed-e8f1-7e92-9ae3-29fda4b019e5" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019fb4ed-e8f1-76b2-89dd-22ca651622e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019fb4ed-e8f1-7851-b94a-586fa95447da" 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_019fb4ed-e8f1-76b2-89dd-22ca651622e4" xlink:to="loc_us-gaap_CommonStockValue_019fb4ed-e8f1-7851-b94a-586fa95447da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019fb4ed-e8f1-76d5-9ee8-c658ae52c54c" 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_019fb4ed-e8f1-76b2-89dd-22ca651622e4" xlink:to="loc_us-gaap_StockholdersEquity_019fb4ed-e8f1-76d5-9ee8-c658ae52c54c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/BalanceSheetsParenthetical" xlink:type="simple" xlink:href="itg-20260630.xsd#BalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/BalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" 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_019fb4ed-e8f1-7555-91df-979e8b4ddac8" 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_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" xlink:to="loc_us-gaap_StatementTable_019fb4ed-e8f1-7555-91df-979e8b4ddac8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019fb4ed-e8f1-7555-91df-979e8b4ddac8" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-77cc-aa54-9054744fa609" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fb4ed-e8f1-70d3-9691-8aff22cd8ae3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-77cc-aa54-9054744fa609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_019fb4ed-e8f1-77ce-98ae-6d283942f326" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019fb4ed-e8f1-77cc-aa54-9054744fa609" xlink:to="loc_srt_ParentCompanyMember_019fb4ed-e8f1-77ce-98ae-6d283942f326" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fb4ed-e8f1-7173-a48e-7fc2358ecf77" 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_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fb4ed-e8f1-7173-a48e-7fc2358ecf77" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019fcedb-eee9-7b00-8cb6-d3252388d8ec" 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_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019fcedb-eee9-7b00-8cb6-d3252388d8ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019fb4ed-e8f1-73c6-8081-5bb1091223bf" 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_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" xlink:to="loc_us-gaap_CommonStockSharesIssued_019fb4ed-e8f1-73c6-8081-5bb1091223bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019fb4ed-e8f1-7067-aac5-89d3d8714921" 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_019fb4ed-e8f1-72f8-9cb1-979eccf336d7" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019fb4ed-e8f1-7067-aac5-89d3d8714921" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#SummaryofSignificantAccountingPoliciesDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019fd857-dfea-7297-8c13-490c29737557" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019fd857-dfea-7297-8c13-490c29737557" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fd857-dfea-7297-8c13-490c29737557_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fd857-dfea-7297-8c13-490c29737557" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fd857-dfea-7297-8c13-490c29737557_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fd857-dfea-7711-a5c4-fa38620047b3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fd857-dfea-7297-8c13-490c29737557" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fd857-dfea-7711-a5c4-fa38620047b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_019fd857-dfea-7e59-8e6f-9c692b0b509e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019fd857-dfea-7711-a5c4-fa38620047b3" xlink:to="loc_srt_ParentCompanyMember_019fd857-dfea-7e59-8e6f-9c692b0b509e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fd857-dfea-7e52-95d8-57666ab75ec0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fd857-dfea-7e52-95d8-57666ab75ec0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fd857-dfea-7e52-95d8-57666ab75ec0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fd857-dfea-7e52-95d8-57666ab75ec0" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fd857-dfea-7e52-95d8-57666ab75ec0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fd857-dfea-7d5b-a69a-93b562f1450d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fd857-dfea-7e52-95d8-57666ab75ec0" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fd857-dfea-7d5b-a69a-93b562f1450d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fd857-dfea-7239-851e-8452b9706a95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fd857-dfea-7d5b-a69a-93b562f1450d" xlink:to="loc_us-gaap_SubsequentEventMember_019fd857-dfea-7239-851e-8452b9706a95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019fd857-dfea-7271-9e79-089f25e25adc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019fd857-dfea-7271-9e79-089f25e25adc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fd857-dfea-7271-9e79-089f25e25adc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fd857-dfea-7271-9e79-089f25e25adc" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fd857-dfea-7271-9e79-089f25e25adc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fd857-dfea-7c93-85bc-a722ffe77e1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fd857-dfea-7271-9e79-089f25e25adc" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fd857-dfea-7c93-85bc-a722ffe77e1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_019fd857-dfea-723e-bf46-baf5912c50b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fd857-dfea-7c93-85bc-a722ffe77e1b" xlink:to="loc_us-gaap_IPOMember_019fd857-dfea-723e-bf46-baf5912c50b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019fd857-dfea-7c07-9582-b7e0d97c68be" xlink:to="loc_srt_OwnershipAxis_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f" xlink:to="loc_srt_OwnershipDomain_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fd857-dfea-797c-964e-5eb0404316b7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_019fd857-dfea-73f7-b7fd-0cea5c9f1c1f" xlink:to="loc_srt_OwnershipDomain_019fd857-dfea-797c-964e-5eb0404316b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ITGParentMember_019fd857-dfea-7302-84c7-f429fef7f366" xlink:href="itg-20260630.xsd#itg_ITGParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_019fd857-dfea-797c-964e-5eb0404316b7" xlink:to="loc_itg_ITGParentMember_019fd857-dfea-7302-84c7-f429fef7f366" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fd857-dfea-7bac-a33a-1c553aee3460" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fd857-dfea-7bac-a33a-1c553aee3460" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019fd857-dfea-76f9-b6f2-4628299e4626" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019fd857-dfea-76f9-b6f2-4628299e4626" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019fd857-dfea-7ad3-af12-4400d24c8174" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019fd857-dfea-7ad3-af12-4400d24c8174" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalanceShares_019fd857-dfea-7fc1-86ed-cb0dab4ebc11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentOwnedBalanceShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_InvestmentOwnedBalanceShares_019fd857-dfea-7fc1-86ed-cb0dab4ebc11" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fd857-dfea-78cb-b604-b50734d274cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fd857-dfea-7fd7-b349-b7456a945eaa" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fd857-dfea-78cb-b604-b50734d274cc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/StockholdersEquityDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#StockholdersEquityDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019fce47-1dbf-77fe-8a2f-93e123374eb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019fce47-1dbf-77fe-8a2f-93e123374eb4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019fce47-1dbf-77fe-8a2f-93e123374eb4" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fce47-1dbf-7ec9-9c95-1db79865399a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fce47-1dbf-7d71-bbfe-b1a8cd3cb2e3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fce47-1dbf-7ec9-9c95-1db79865399a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_019fce47-8d4c-7785-82cf-47fda736e4a9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019fce47-1dbf-7ec9-9c95-1db79865399a" xlink:to="loc_srt_ParentCompanyMember_019fce47-8d4c-7785-82cf-47fda736e4a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019fce43-6046-7bb3-b80c-bbe9d8829df6" 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_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019fce43-6046-7bb3-b80c-bbe9d8829df6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fce47-1dbf-74cc-b5aa-5372dfb3b4a3" 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_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fce47-1dbf-74cc-b5aa-5372dfb3b4a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fce48-2876-790e-8834-90a9f87491f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fce48-2876-790e-8834-90a9f87491f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019fce48-525f-7fc4-954b-f1a8667d9ed8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019fce47-1dbf-7573-b69f-5cf42338476b" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019fce48-525f-7fc4-954b-f1a8667d9ed8" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019fce60-cf82-7d28-8559-a106359b708f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019fce60-cf82-7c0b-8e73-2ebbeeb2aee3" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019fce60-cf82-7d28-8559-a106359b708f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_019fce60-e424-766e-8f31-9ab6866a2e5a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019fce60-cf82-7d28-8559-a106359b708f" xlink:to="loc_srt_ParentCompanyMember_019fce60-e424-766e-8f31-9ab6866a2e5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fce61-0658-725f-a4c0-51fa31cd93b7" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fce61-0658-725f-a4c0-51fa31cd93b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fce61-0658-725f-a4c0-51fa31cd93b7_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_019fce61-0658-725f-a4c0-51fa31cd93b7" xlink:to="loc_us-gaap_ClassOfStockDomain_019fce61-0658-725f-a4c0-51fa31cd93b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fce61-0658-745e-a664-ee30ab748f98" 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_019fce61-0658-725f-a4c0-51fa31cd93b7" xlink:to="loc_us-gaap_ClassOfStockDomain_019fce61-0658-745e-a664-ee30ab748f98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019fce61-364c-79d2-86f9-72e0390ec23c" 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_019fce61-0658-745e-a664-ee30ab748f98" xlink:to="loc_us-gaap_CommonClassBMember_019fce61-364c-79d2-86f9-72e0390ec23c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019fce62-e598-7b03-8a9e-919fb558cd71" 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_019fce61-0658-745e-a664-ee30ab748f98" xlink:to="loc_us-gaap_CommonClassAMember_019fce62-e598-7b03-8a9e-919fb558cd71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fce61-560e-7990-aa41-d8b7f31cbf54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fce61-560e-7990-aa41-d8b7f31cbf54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fce61-560e-7990-aa41-d8b7f31cbf54_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fce61-560e-7990-aa41-d8b7f31cbf54" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fce61-560e-7990-aa41-d8b7f31cbf54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fce61-560e-786c-a321-b22adb30218b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fce61-560e-7990-aa41-d8b7f31cbf54" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fce61-560e-786c-a321-b22adb30218b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fce61-6910-75fb-bc4a-ddf3a15385ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fce61-560e-786c-a321-b22adb30218b" xlink:to="loc_us-gaap_SubsequentEventMember_019fce61-6910-75fb-bc4a-ddf3a15385ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce61-cd16-7cb5-86fa-9f3cd10835d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce61-cd16-7cb5-86fa-9f3cd10835d9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce61-cd16-7cb5-86fa-9f3cd10835d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce61-cd16-7cb5-86fa-9f3cd10835d9" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce61-cd16-7cb5-86fa-9f3cd10835d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce61-cd16-7200-a56c-e846d37800a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce61-cd16-7cb5-86fa-9f3cd10835d9" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce61-cd16-7200-a56c-e846d37800a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019fce61-ea5a-7835-b6c0-bbaa26785abd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce61-cd16-7200-a56c-e846d37800a5" xlink:to="loc_us-gaap_PrivatePlacementMember_019fce61-ea5a-7835-b6c0-bbaa26785abd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fce63-4e13-744e-905f-7be99867b39c" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fce63-4e13-744e-905f-7be99867b39c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce63-4e13-744e-905f-7be99867b39c_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_019fce63-4e13-744e-905f-7be99867b39c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce63-4e13-744e-905f-7be99867b39c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce63-4e13-7c82-9dc8-965daaf45509" 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_019fce63-4e13-744e-905f-7be99867b39c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce63-4e13-7c82-9dc8-965daaf45509" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_OaktreeBlockedFundMember_019fce64-11d0-76d0-a62f-51486ceb4485" xlink:href="itg-20260630.xsd#itg_OaktreeBlockedFundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce63-4e13-7c82-9dc8-965daaf45509" xlink:to="loc_itg_OaktreeBlockedFundMember_019fce64-11d0-76d0-a62f-51486ceb4485" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_019fceba-fca2-7945-954d-310e4dd270db" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_srt_OwnershipAxis_019fceba-fca2-7945-954d-310e4dd270db" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fceba-fca2-7945-954d-310e4dd270db_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_019fceba-fca2-7945-954d-310e4dd270db" xlink:to="loc_srt_OwnershipDomain_019fceba-fca2-7945-954d-310e4dd270db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fceba-fca2-7161-a445-00dab7a24ebb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_019fceba-fca2-7945-954d-310e4dd270db" xlink:to="loc_srt_OwnershipDomain_019fceba-fca2-7161-a445-00dab7a24ebb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ITGParentMember_019fcebb-1632-7c61-beba-84b42a2360b8" xlink:href="itg-20260630.xsd#itg_ITGParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_019fceba-fca2-7161-a445-00dab7a24ebb" xlink:to="loc_itg_ITGParentMember_019fcebb-1632-7c61-beba-84b42a2360b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019fcec1-d3ac-78f6-b1c5-11ba73116cf7" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_AwardTypeAxis_019fcec1-d3ac-78f6-b1c5-11ba73116cf7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fcec1-d3ac-78f6-b1c5-11ba73116cf7_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_019fcec1-d3ac-78f6-b1c5-11ba73116cf7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fcec1-d3ac-78f6-b1c5-11ba73116cf7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fcec1-d3ac-7bb1-afc8-0ac9e153e09d" 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_019fcec1-d3ac-78f6-b1c5-11ba73116cf7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fcec1-d3ac-7bb1-afc8-0ac9e153e09d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019fcec1-e6fa-7e38-97cf-4735d7dcc495" 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_019fcec1-d3ac-7bb1-afc8-0ac9e153e09d" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019fcec1-e6fa-7e38-97cf-4735d7dcc495" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019fe90d-a98d-7c80-9e42-3bf0bce57bf2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fcec1-d3ac-7bb1-afc8-0ac9e153e09d" xlink:to="loc_us-gaap_PerformanceSharesMember_019fe90d-a98d-7c80-9e42-3bf0bce57bf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019fcec2-1e60-78fd-b636-084ce595267a" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_PlanNameAxis_019fcec2-1e60-78fd-b636-084ce595267a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019fcec2-1e60-78fd-b636-084ce595267a_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_019fcec2-1e60-78fd-b636-084ce595267a" xlink:to="loc_us-gaap_PlanNameDomain_019fcec2-1e60-78fd-b636-084ce595267a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019fcec2-1e60-7fb8-b1f6-37224b9538c7" 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_019fcec2-1e60-78fd-b636-084ce595267a" xlink:to="loc_us-gaap_PlanNameDomain_019fcec2-1e60-7fb8-b1f6-37224b9538c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_A2026IncentiveAwardPlanMember_019fcec2-4db7-7b0c-a009-5972b7c5e318" xlink:href="itg-20260630.xsd#itg_A2026IncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019fcec2-1e60-7fb8-b1f6-37224b9538c7" xlink:to="loc_itg_A2026IncentiveAwardPlanMember_019fcec2-4db7-7b0c-a009-5972b7c5e318" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019fcec2-f409-7a99-b1ba-89502e4f35a8" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_srt_RangeAxis_019fcec2-f409-7a99-b1ba-89502e4f35a8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fcec2-f409-7a99-b1ba-89502e4f35a8_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_019fcec2-f409-7a99-b1ba-89502e4f35a8" xlink:to="loc_srt_RangeMember_019fcec2-f409-7a99-b1ba-89502e4f35a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fcec2-f409-78a8-bbbe-cb2709d40f33" 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_019fcec2-f409-7a99-b1ba-89502e4f35a8" xlink:to="loc_srt_RangeMember_019fcec2-f409-78a8-bbbe-cb2709d40f33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019fcec3-0f65-7b77-b061-0d89a19d8a38" 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_019fcec2-f409-78a8-bbbe-cb2709d40f33" xlink:to="loc_srt_MinimumMember_019fcec3-0f65-7b77-b061-0d89a19d8a38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019fcec3-261c-7742-b81e-2410b91406af" 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_019fcec2-f409-78a8-bbbe-cb2709d40f33" xlink:to="loc_srt_MaximumMember_019fcec3-261c-7742-b81e-2410b91406af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019fceca-760f-7859-943f-90c47c47bb06" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_dei_LegalEntityAxis_019fceca-760f-7859-943f-90c47c47bb06" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fceca-760f-7859-943f-90c47c47bb06_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019fceca-760f-7859-943f-90c47c47bb06" xlink:to="loc_dei_EntityDomain_019fceca-760f-7859-943f-90c47c47bb06_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fceca-760f-75f5-8d27-d4371827f2da" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019fceca-760f-7859-943f-90c47c47bb06" xlink:to="loc_dei_EntityDomain_019fceca-760f-75f5-8d27-d4371827f2da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassBCommonStockholdersMember_019fcecb-0200-7d81-88cf-b0541dc95dd0" xlink:href="itg-20260630.xsd#itg_ClassBCommonStockholdersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019fceca-760f-75f5-8d27-d4371827f2da" xlink:to="loc_itg_ClassBCommonStockholdersMember_019fcecb-0200-7d81-88cf-b0541dc95dd0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019fe90e-8a03-7466-a566-8d52decf3e75" 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_SubsequentEventTable_019fce60-99a0-7671-a747-22ca3f2fa559" xlink:to="loc_us-gaap_VestingAxis_019fe90e-8a03-7466-a566-8d52decf3e75" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019fe90e-8a03-7466-a566-8d52decf3e75_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_019fe90e-8a03-7466-a566-8d52decf3e75" xlink:to="loc_us-gaap_VestingDomain_019fe90e-8a03-7466-a566-8d52decf3e75_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019fe90e-8a03-7269-b6a9-3c563f263ca0" 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_019fe90e-8a03-7466-a566-8d52decf3e75" xlink:to="loc_us-gaap_VestingDomain_019fe90e-8a03-7269-b6a9-3c563f263ca0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019fe90f-74e2-7ac1-93f2-2a942d370fe0" 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_019fe90e-8a03-7269-b6a9-3c563f263ca0" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019fe90f-74e2-7ac1-93f2-2a942d370fe0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019fe90f-b487-7efc-a8a9-d0f5336b2343" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019fe90e-8a03-7269-b6a9-3c563f263ca0" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019fe90f-b487-7efc-a8a9-d0f5336b2343" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fce60-99a0-7a57-a9ca-499c40764485" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fce60-99a0-7a57-a9ca-499c40764485" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019fce62-a358-7aef-9445-968901a1a7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019fce62-a358-7aef-9445-968901a1a7e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fd85a-da81-7242-a2b6-fe9ec30bc80d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fd85a-da81-7242-a2b6-fe9ec30bc80d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_RequiredRatioOfCommonStockToInvestmentOwnedShares_019fce91-869a-7b9f-a2a6-7313a4a287ad" xlink:href="itg-20260630.xsd#itg_RequiredRatioOfCommonStockToInvestmentOwnedShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_RequiredRatioOfCommonStockToInvestmentOwnedShares_019fce91-869a-7b9f-a2a6-7313a4a287ad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019fcea7-5e10-7dd4-8e8e-444a65b6e426" 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_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019fcea7-5e10-7dd4-8e8e-444a65b6e426" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fcea8-55f9-7d80-b1fe-aeda2fa4a03c" 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_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019fcea8-55f9-7d80-b1fe-aeda2fa4a03c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019fcea9-1777-7db3-b6b0-76323ea46134" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019fcea9-1777-7db3-b6b0-76323ea46134" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_NumberOfBoardOfDirectorsClasses_019fceb2-323e-7a06-b2ed-77f456b5358a" xlink:href="itg-20260630.xsd#itg_NumberOfBoardOfDirectorsClasses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_NumberOfBoardOfDirectorsClasses_019fceb2-323e-7a06-b2ed-77f456b5358a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CommonStockNumberOfVotesPerCommonShare_019fceae-8cf7-7d80-9595-c8cd269384eb" xlink:href="itg-20260630.xsd#itg_CommonStockNumberOfVotesPerCommonShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_CommonStockNumberOfVotesPerCommonShare_019fceae-8cf7-7d80-9595-c8cd269384eb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_RequiredRatioToIssueCommonStockInvestmentSharesOwnedAndCommonStockHeldByExistingOwners_019fceb3-cd2d-71b7-8b06-44e6347f3bf1" xlink:href="itg-20260630.xsd#itg_RequiredRatioToIssueCommonStockInvestmentSharesOwnedAndCommonStockHeldByExistingOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_RequiredRatioToIssueCommonStockInvestmentSharesOwnedAndCommonStockHeldByExistingOwners_019fceb3-cd2d-71b7-8b06-44e6347f3bf1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_StockCancelationRatioUponRedemptionElectionOfInvestmentSharesOwnedByExistingOwners_019fceb6-ea0a-7bb7-bf15-3028045a4604" xlink:href="itg-20260630.xsd#itg_StockCancelationRatioUponRedemptionElectionOfInvestmentSharesOwnedByExistingOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_StockCancelationRatioUponRedemptionElectionOfInvestmentSharesOwnedByExistingOwners_019fceb6-ea0a-7bb7-bf15-3028045a4604" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_TaxReceivableAgreementPercentageOfTaxBenefitsDueToEquityOwners_019fcebf-3e81-79d0-bace-adb7a0004348" xlink:href="itg-20260630.xsd#itg_TaxReceivableAgreementPercentageOfTaxBenefitsDueToEquityOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_TaxReceivableAgreementPercentageOfTaxBenefitsDueToEquityOwners_019fcebf-3e81-79d0-bace-adb7a0004348" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019fe90c-81fc-7822-8589-e112f7b3c372" 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_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019fe90c-81fc-7822-8589-e112f7b3c372" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019fcec1-2b8e-7c98-a2eb-a45406223c3d" 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_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019fcec1-2b8e-7c98-a2eb-a45406223c3d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019fcec1-5aa8-760d-8368-03432c1e8b41" 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_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019fcec1-5aa8-760d-8368-03432c1e8b41" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_MaximumNumberOfDirectorsNominationsAvailableUponEquityOwnershipThreshold_019fcec4-929d-72e1-b5fc-c74e8ca60d95" xlink:href="itg-20260630.xsd#itg_MaximumNumberOfDirectorsNominationsAvailableUponEquityOwnershipThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_MaximumNumberOfDirectorsNominationsAvailableUponEquityOwnershipThreshold_019fcec4-929d-72e1-b5fc-c74e8ca60d95" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_MinimumEquityOwnershipThresholdTriggeringEntitledDirectorNomination_019fcec5-03cb-7f26-9ac5-499d28833e2b" xlink:href="itg-20260630.xsd#itg_MinimumEquityOwnershipThresholdTriggeringEntitledDirectorNomination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_MinimumEquityOwnershipThresholdTriggeringEntitledDirectorNomination_019fcec5-03cb-7f26-9ac5-499d28833e2b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_MinimumEquityOwnershipThresholdTriggeringCorporateActionRequiredWrittenConsent_019fcec5-78c9-711e-acd0-c3e89c00d480" xlink:href="itg-20260630.xsd#itg_MinimumEquityOwnershipThresholdTriggeringCorporateActionRequiredWrittenConsent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce60-99a0-7c4c-8d7b-ac88af7a7486" xlink:to="loc_itg_MinimumEquityOwnershipThresholdTriggeringCorporateActionRequiredWrittenConsent_019fcec5-78c9-711e-acd0-c3e89c00d480" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/ORGANIZATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNarrativeDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#ORGANIZATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNarrativeDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/ORGANIZATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019fe92f-bba7-7576-aa09-e7580bc9609a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019fe92f-bba7-71e1-8fd9-db6c7a560026" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fe92f-bba7-7576-aa09-e7580bc9609a" xlink:to="loc_us-gaap_ConcentrationRiskTable_019fe92f-bba7-71e1-8fd9-db6c7a560026" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fe92f-bba7-7953-8df9-09bfe1f66d94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fe92f-bba7-71e1-8fd9-db6c7a560026" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fe92f-bba7-7953-8df9-09bfe1f66d94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fe92f-bba7-7953-8df9-09bfe1f66d94_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fe92f-bba7-7953-8df9-09bfe1f66d94" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fe92f-bba7-7953-8df9-09bfe1f66d94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fe92f-bba7-7e6f-b1cd-367c7cd8970d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fe92f-bba7-7953-8df9-09bfe1f66d94" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fe92f-bba7-7e6f-b1cd-367c7cd8970d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019fe930-eb1b-7a3c-97dc-94cda2f37da7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fe92f-bba7-7e6f-b1cd-367c7cd8970d" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019fe930-eb1b-7a3c-97dc-94cda2f37da7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019fe930-0538-766b-b89b-fd4c2a793a02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fe92f-bba7-71e1-8fd9-db6c7a560026" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019fe930-0538-766b-b89b-fd4c2a793a02" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019fe930-0538-766b-b89b-fd4c2a793a02_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019fe930-0538-766b-b89b-fd4c2a793a02" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019fe930-0538-766b-b89b-fd4c2a793a02_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019fe930-0538-77e2-a680-898cbeefe935" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019fe930-0538-766b-b89b-fd4c2a793a02" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019fe930-0538-77e2-a680-898cbeefe935" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_TimingOfTransferOfGoodOrServiceMember_019fe935-8f04-7219-a4a2-135424340601" xlink:href="itg-20260630.xsd#itg_TimingOfTransferOfGoodOrServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019fe930-0538-77e2-a680-898cbeefe935" xlink:to="loc_itg_TimingOfTransferOfGoodOrServiceMember_019fe935-8f04-7219-a4a2-135424340601" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019fe930-5171-74bc-b081-c49941e0b45f" 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_ConcentrationRiskTable_019fe92f-bba7-71e1-8fd9-db6c7a560026" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019fe930-5171-74bc-b081-c49941e0b45f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019fe930-5171-74bc-b081-c49941e0b45f_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_019fe930-5171-74bc-b081-c49941e0b45f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019fe930-5171-74bc-b081-c49941e0b45f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019fe930-5171-7364-a168-c688789343b8" 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_019fe930-5171-74bc-b081-c49941e0b45f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019fe930-5171-7364-a168-c688789343b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019fe933-a81a-7cdb-98d4-3d53d48caba6" 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_019fe930-5171-7364-a168-c688789343b8" xlink:to="loc_us-gaap_TransferredOverTimeMember_019fe933-a81a-7cdb-98d4-3d53d48caba6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019fe932-c580-783b-a281-a5e99a14de06" 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_019fe930-5171-7364-a168-c688789343b8" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019fe932-c580-783b-a281-a5e99a14de06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019fe92f-bba7-7260-8f9f-4771ea928ec6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fe92f-bba7-7576-aa09-e7580bc9609a" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019fe92f-bba7-7260-8f9f-4771ea928ec6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019fc537-72a3-75f1-95af-ca3f2fe85106" 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_ConcentrationRiskLineItems_019fe92f-bba7-7576-aa09-e7580bc9609a" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019fc537-72a3-75f1-95af-ca3f2fe85106" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#BUSINESSCOMBINATIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" 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_019fc8f2-d580-71c3-9c2c-90212b80e4ed" 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_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fc8f2-d580-71c3-9c2c-90212b80e4ed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fc8f5-678f-7260-baee-fbb7ed9f76f9" 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_019fc8f2-d580-71c3-9c2c-90212b80e4ed" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fc8f5-678f-7260-baee-fbb7ed9f76f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-7260-baee-fbb7ed9f76f9_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_019fc8f5-678f-7260-baee-fbb7ed9f76f9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-7260-baee-fbb7ed9f76f9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" 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_019fc8f5-678f-7260-baee-fbb7ed9f76f9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_TilsonMember_019fc8fa-95a0-7f79-a2f8-29bd942d696e" xlink:href="itg-20260630.xsd#itg_TilsonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:to="loc_itg_TilsonMember_019fc8fa-95a0-7f79-a2f8-29bd942d696e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_VisionMember_019fc8f7-c610-76a4-82b8-aa5d03274d93" xlink:href="itg-20260630.xsd#itg_VisionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:to="loc_itg_VisionMember_019fc8f7-c610-76a4-82b8-aa5d03274d93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_QuasarMember_019fc8fd-c886-70a0-a683-19e6f59c5679" xlink:href="itg-20260630.xsd#itg_QuasarMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:to="loc_itg_QuasarMember_019fc8fd-c886-70a0-a683-19e6f59c5679" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_AdvantageMember_019fc8fe-9672-7529-80aa-fade8bea1689" xlink:href="itg-20260630.xsd#itg_AdvantageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:to="loc_itg_AdvantageMember_019fc8fe-9672-7529-80aa-fade8bea1689" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019fc909-77e9-784d-94d5-6c43cd19c1e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc8f5-678f-73d6-bf2b-44c3bd7ac66d" xlink:to="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019fc909-77e9-784d-94d5-6c43cd19c1e0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_IndividuallyImmaterialBusinessCombinationOneMember_019fc91a-bcfd-7e19-ac16-a1da52c080f8" xlink:href="itg-20260630.xsd#itg_IndividuallyImmaterialBusinessCombinationOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019fc909-77e9-784d-94d5-6c43cd19c1e0" xlink:to="loc_itg_IndividuallyImmaterialBusinessCombinationOneMember_019fc91a-bcfd-7e19-ac16-a1da52c080f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_IndividuallyImmaterialBusinessCombinationTwoMember_019fc91a-edd7-7c6b-b98c-21c04fe2ca5b" xlink:href="itg-20260630.xsd#itg_IndividuallyImmaterialBusinessCombinationTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019fc909-77e9-784d-94d5-6c43cd19c1e0" xlink:to="loc_itg_IndividuallyImmaterialBusinessCombinationTwoMember_019fc91a-edd7-7c6b-b98c-21c04fe2ca5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationByTypeAxis_019fc917-79ce-7e09-a946-a6509ecc706d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContingentConsiderationByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fc8f2-d580-71c3-9c2c-90212b80e4ed" xlink:to="loc_us-gaap_ContingentConsiderationByTypeAxis_019fc917-79ce-7e09-a946-a6509ecc706d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7e09-a946-a6509ecc706d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContingentConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContingentConsiderationByTypeAxis_019fc917-79ce-7e09-a946-a6509ecc706d" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7e09-a946-a6509ecc706d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7c1e-89b8-99d2c0b1d2e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContingentConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContingentConsiderationByTypeAxis_019fc917-79ce-7e09-a946-a6509ecc706d" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7c1e-89b8-99d2c0b1d2e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_AchievementOfCertainRevenueAndGrossMarginCriteriaMember_019fc91b-effc-70e3-bb9f-ce7969e95f6a" xlink:href="itg-20260630.xsd#itg_AchievementOfCertainRevenueAndGrossMarginCriteriaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7c1e-89b8-99d2c0b1d2e8" xlink:to="loc_itg_AchievementOfCertainRevenueAndGrossMarginCriteriaMember_019fc91b-effc-70e3-bb9f-ce7969e95f6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_AnnualRevenueIncreaseMember_019fc91c-43c1-73cc-869e-fa4d0442b1a6" xlink:href="itg-20260630.xsd#itg_AnnualRevenueIncreaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_019fc917-79ce-7c1e-89b8-99d2c0b1d2e8" xlink:to="loc_itg_AnnualRevenueIncreaseMember_019fc91c-43c1-73cc-869e-fa4d0442b1a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019fc8f2-d580-7afb-b71c-07e63ff390b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019fc8f2-d580-7afb-b71c-07e63ff390b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfProductiveAssets_019fc8fa-4e70-7f2a-a2b5-b85b6584823d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSaleOfProductiveAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_ProceedsFromSaleOfProductiveAssets_019fc8fa-4e70-7f2a-a2b5-b85b6584823d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019fc8f3-2698-7c54-8fad-f6b2f875d71f" 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_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_Goodwill_019fc8f3-2698-7c54-8fad-f6b2f875d71f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019fc8f4-6366-7cba-a39f-e03a4339466e" 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_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019fc8f4-6366-7cba-a39f-e03a4339466e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019fc8f4-d33d-701b-b4c7-a19c51d647c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019fc8f4-d33d-701b-b4c7-a19c51d647c4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019fc8f5-35f8-77ce-bf9b-b3a9de2c8d54" 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_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019fc8f5-35f8-77ce-bf9b-b3a9de2c8d54" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_019fc8f8-7e2e-704f-a389-c15ca47ab802" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_019fc8f8-7e2e-704f-a389-c15ca47ab802" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationRecognizedLiabilityAssumedAccruedLiabilitiesCurrent_019fc8f9-0fb0-7f0d-845c-a4583e2c2dc1" xlink:href="itg-20260630.xsd#itg_BusinessCombinationRecognizedLiabilityAssumedAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_itg_BusinessCombinationRecognizedLiabilityAssumedAccruedLiabilitiesCurrent_019fc8f9-0fb0-7f0d-845c-a4583e2c2dc1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_019fc8f6-22df-7505-b70e-4adfd4ca7e50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_019fc8f6-22df-7505-b70e-4adfd4ca7e50" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019fc8ff-704d-704b-a045-ef087651ac17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019fc8ff-704d-704b-a045-ef087651ac17" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationContingentConsiderationPercentageOfAdjustedEBITDA_019fc902-2c95-7cdf-aac4-b6008b1648ab" xlink:href="itg-20260630.xsd#itg_BusinessCombinationContingentConsiderationPercentageOfAdjustedEBITDA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_itg_BusinessCombinationContingentConsiderationPercentageOfAdjustedEBITDA_019fc902-2c95-7cdf-aac4-b6008b1648ab" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationContingentConsiderationPeriod_019fc902-b4bb-7596-9891-97c46893b339" xlink:href="itg-20260630.xsd#itg_BusinessCombinationContingentConsiderationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_itg_BusinessCombinationContingentConsiderationPeriod_019fc902-b4bb-7596-9891-97c46893b339" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_019fc908-895b-766a-b28f-b307db4df1b9" 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_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_019fc908-895b-766a-b28f-b307db4df1b9" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationContingentConsiderationPercentageOfAnnualRevenueIncrease_019fc918-efa1-7bdb-8bc8-bd183561e696" xlink:href="itg-20260630.xsd#itg_BusinessCombinationContingentConsiderationPercentageOfAnnualRevenueIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_itg_BusinessCombinationContingentConsiderationPercentageOfAnnualRevenueIncrease_019fc918-efa1-7bdb-8bc8-bd183561e696" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationContingentConsiderationRangeOfOutcomesMaximumAnnualAmount_019fc919-cc79-74cd-aa41-5a034d99c5af" xlink:href="itg-20260630.xsd#itg_BusinessCombinationContingentConsiderationRangeOfOutcomesMaximumAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc8f2-d580-7e9f-aa9e-7986f2bd753c" xlink:to="loc_itg_BusinessCombinationContingentConsiderationRangeOfOutcomesMaximumAnnualAmount_019fc919-cc79-74cd-aa41-5a034d99c5af" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019fb4ed-e8f1-7cef-b26b-c45b2728f78f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019fb4ed-e8f1-7b21-918a-b08f26cc8461" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019fb4ed-e8f1-7cef-b26b-c45b2728f78f" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019fb4ed-e8f1-7b21-918a-b08f26cc8461" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019fb4ed-e8f1-760e-b86b-6b7b7e211956" 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_ScheduleOfPropertyPlantAndEquipmentTable_019fb4ed-e8f1-7b21-918a-b08f26cc8461" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019fb4ed-e8f1-760e-b86b-6b7b7e211956" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-760e-b86b-6b7b7e211956_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_019fb4ed-e8f1-760e-b86b-6b7b7e211956" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-760e-b86b-6b7b7e211956_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" 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_019fb4ed-e8f1-760e-b86b-6b7b7e211956" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019fb4ed-e8f1-761b-9fc7-b10dec65cf2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019fb4ed-e8f1-761b-9fc7-b10dec65cf2c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_019fb4ed-e8f1-7250-b4d6-b6e059af8f21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_019fb4ed-e8f1-7250-b4d6-b6e059af8f21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019fb4ed-e8f1-7c09-a07b-fe159dd85a59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019fb4ed-e8f1-7c09-a07b-fe159dd85a59" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019fb4ed-e8f1-7cf6-bbc2-af86fa29ac5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019fb4ed-e8f1-7829-b22a-08d9ff759101" xlink:to="loc_us-gaap_ComputerEquipmentMember_019fb4ed-e8f1-7cf6-bbc2-af86fa29ac5a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019fb4ed-e8f1-77df-bdbe-26040a4ee658" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019fb4ed-e8f1-7cef-b26b-c45b2728f78f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019fb4ed-e8f1-77df-bdbe-26040a4ee658" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019fb4ed-e8f1-7680-9c23-777e03796793" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019fb4ed-e8f1-7cef-b26b-c45b2728f78f" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019fb4ed-e8f1-7680-9c23-777e03796793" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019fb4ed-e8f1-755f-b34f-271f252250ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019fb4ed-e8f1-7cef-b26b-c45b2728f78f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019fb4ed-e8f1-755f-b34f-271f252250ce" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/INTANGIBLEASSETSScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#INTANGIBLEASSETSScheduleofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/INTANGIBLEASSETSScheduleofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019fb4ed-e8f1-7d40-a3b9-dc83e67fa9f5" 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_019fb4ed-e8f1-77b5-bf25-3b3a94b8bf1c" 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_019fb4ed-e8f1-7d40-a3b9-dc83e67fa9f5" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019fb4ed-e8f1-77b5-bf25-3b3a94b8bf1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec" 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_019fb4ed-e8f1-77b5-bf25-3b3a94b8bf1c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec_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_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7a20-927b-f144357bde2c" 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_019fb4ed-e8f1-7380-a7c9-4765eb77f3ec" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7a20-927b-f144357bde2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019fb4ed-e8f1-72bb-a66e-e46ec0c974bd" 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_019fb4ed-e8f1-7a20-927b-f144357bde2c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019fb4ed-e8f1-72bb-a66e-e46ec0c974bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_019fb4ed-e8f1-7dd2-915d-3db56adbab70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TrademarksAndTradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7a20-927b-f144357bde2c" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_019fb4ed-e8f1-7dd2-915d-3db56adbab70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_019fb4ed-e8f1-7544-bee8-c53bb82564bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fb4ed-e8f1-7a20-927b-f144357bde2c" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_019fb4ed-e8f1-7544-bee8-c53bb82564bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019fb4ed-e8f1-7058-81bd-b9915a317549" 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_019fb4ed-e8f1-7a20-927b-f144357bde2c" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019fb4ed-e8f1-7058-81bd-b9915a317549" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019fb4ed-e8f1-7426-a77b-ce244a76d896" 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_019fb4ed-e8f1-7d40-a3b9-dc83e67fa9f5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019fb4ed-e8f1-7426-a77b-ce244a76d896" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019fb4ed-e8f1-72a8-9654-28de0bae81a0" 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_019fb4ed-e8f1-7d40-a3b9-dc83e67fa9f5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019fb4ed-e8f1-72a8-9654-28de0bae81a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019fb4ed-e8f1-7943-9716-5f57225b1012" 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_019fb4ed-e8f1-7d40-a3b9-dc83e67fa9f5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019fb4ed-e8f1-7943-9716-5f57225b1012" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/DEBTScheduleofLongTermDebtDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#DEBTScheduleofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/DEBTScheduleofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019fc927-8b4d-7024-803e-9b9953df61fb" 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_019fc927-8b4d-7dc9-8ea4-d4ebaeaabdf9" 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_019fc927-8b4d-7024-803e-9b9953df61fb" xlink:to="loc_us-gaap_DebtInstrumentTable_019fc927-8b4d-7dc9-8ea4-d4ebaeaabdf9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca" 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_019fc927-8b4d-7dc9-8ea4-d4ebaeaabdf9" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca_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_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc929-acd6-7fce-843a-f7e3e9641896" 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_019fc929-acd6-7b2f-8a7d-0bfc9cf6ccca" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc929-acd6-7fce-843a-f7e3e9641896" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CreditAgreementMember_019fc929-da47-7009-8051-a09c960857e8" xlink:href="itg-20260630.xsd#itg_CreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc929-acd6-7fce-843a-f7e3e9641896" xlink:to="loc_itg_CreditAgreementMember_019fc929-da47-7009-8051-a09c960857e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fc929-fa1f-7e03-a4c5-05d4f07722a2" 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_019fc927-8b4d-7dc9-8ea4-d4ebaeaabdf9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fc929-fa1f-7e03-a4c5-05d4f07722a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc929-fa1f-7e03-a4c5-05d4f07722a2_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_019fc929-fa1f-7e03-a4c5-05d4f07722a2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc929-fa1f-7e03-a4c5-05d4f07722a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc929-fa1f-729e-9fe1-272721a6bff8" 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_019fc929-fa1f-7e03-a4c5-05d4f07722a2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc929-fa1f-729e-9fe1-272721a6bff8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019fc92a-190f-76c8-9434-3bda610ae865" 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_019fc929-fa1f-729e-9fe1-272721a6bff8" xlink:to="loc_us-gaap_LineOfCreditMember_019fc92a-190f-76c8-9434-3bda610ae865" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019fc92a-3877-75f6-82dc-d10e05f5f926" 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_DebtInstrumentTable_019fc927-8b4d-7dc9-8ea4-d4ebaeaabdf9" xlink:to="loc_us-gaap_CreditFacilityAxis_019fc92a-3877-75f6-82dc-d10e05f5f926" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc92a-3877-75f6-82dc-d10e05f5f926_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_019fc92a-3877-75f6-82dc-d10e05f5f926" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc92a-3877-75f6-82dc-d10e05f5f926_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc92a-3877-7d00-9b5d-02ffc5e04604" 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_019fc92a-3877-75f6-82dc-d10e05f5f926" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc92a-3877-7d00-9b5d-02ffc5e04604" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019fc92a-5980-7cab-be1b-4482ab1234ce" 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_CreditFacilityDomain_019fc92a-3877-7d00-9b5d-02ffc5e04604" xlink:to="loc_us-gaap_SecuredDebtMember_019fc92a-5980-7cab-be1b-4482ab1234ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019fc92a-793b-759a-a1e5-0b2ea55c5b2f" 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_019fc92a-3877-7d00-9b5d-02ffc5e04604" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019fc92a-793b-759a-a1e5-0b2ea55c5b2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019fc927-8b4d-705f-9ebb-5e6dead2a527" 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_019fc927-8b4d-7024-803e-9b9953df61fb" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019fc927-8b4d-705f-9ebb-5e6dead2a527" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019fc928-7965-7b9b-b773-4e486e155609" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc927-8b4d-7024-803e-9b9953df61fb" xlink:to="loc_us-gaap_LongTermDebtCurrent_019fc928-7965-7b9b-b773-4e486e155609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019fc928-fcbf-7dcb-bff4-c75a612cf7e3" 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_019fc927-8b4d-7024-803e-9b9953df61fb" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019fc928-fcbf-7dcb-bff4-c75a612cf7e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019fc928-b760-7c07-92ce-59abf11985e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc927-8b4d-7024-803e-9b9953df61fb" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019fc928-b760-7c07-92ce-59abf11985e2" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/DEBTNarrativeDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#DEBTNarrativeDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/DEBTNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" 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_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" 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_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_DebtInstrumentTable_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fc937-35a3-74c3-bc8e-935099d1ac8d" 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_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fc937-35a3-74c3-bc8e-935099d1ac8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc937-35a3-74c3-bc8e-935099d1ac8d_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_019fc937-35a3-74c3-bc8e-935099d1ac8d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc937-35a3-74c3-bc8e-935099d1ac8d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc937-35a3-7dd7-9fdc-ca4415b123f0" 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_019fc937-35a3-74c3-bc8e-935099d1ac8d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc937-35a3-7dd7-9fdc-ca4415b123f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019fc938-6cac-7332-86ba-ab4ab27e74ba" 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_019fc937-35a3-7dd7-9fdc-ca4415b123f0" xlink:to="loc_us-gaap_LineOfCreditMember_019fc938-6cac-7332-86ba-ab4ab27e74ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fc937-35a3-7477-a2ab-3e8b6b20608c" 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_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fc937-35a3-7477-a2ab-3e8b6b20608c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7477-a2ab-3e8b6b20608c_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_019fc937-35a3-7477-a2ab-3e8b6b20608c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7477-a2ab-3e8b6b20608c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7985-8b4f-2538c3603622" 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_019fc937-35a3-7477-a2ab-3e8b6b20608c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7985-8b4f-2538c3603622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CreditAgreementMember_019fc938-5096-794a-a6e5-624e954324a0" xlink:href="itg-20260630.xsd#itg_CreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7985-8b4f-2538c3603622" xlink:to="loc_itg_CreditAgreementMember_019fc938-5096-794a-a6e5-624e954324a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CreditAgreementMarch312026UntilDecember312027Member_019fc93d-473a-7e82-b326-6c1ae5afa07c" xlink:href="itg-20260630.xsd#itg_CreditAgreementMarch312026UntilDecember312027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_CreditAgreementMember_019fc938-5096-794a-a6e5-624e954324a0" xlink:to="loc_itg_CreditAgreementMarch312026UntilDecember312027Member_019fc93d-473a-7e82-b326-6c1ae5afa07c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CreditAgreementBeginningMarch312028Member_019fc93d-d1fd-7316-8b71-311fa466a75c" xlink:href="itg-20260630.xsd#itg_CreditAgreementBeginningMarch312028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_CreditAgreementMember_019fc938-5096-794a-a6e5-624e954324a0" xlink:to="loc_itg_CreditAgreementBeginningMarch312028Member_019fc93d-d1fd-7316-8b71-311fa466a75c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7985-8b4f-2538c3603622" xlink:to="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementJune302024UntilDecember312024Member_019fc956-9e5d-7f90-ab45-7d627074c799" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementJune302024UntilDecember312024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:to="loc_itg_PreviousCreditAgreementJune302024UntilDecember312024Member_019fc956-9e5d-7f90-ab45-7d627074c799" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementJune302025UntilDecember312026Member_019fc957-9c66-7bbe-9464-a073703bcdbf" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementJune302025UntilDecember312026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:to="loc_itg_PreviousCreditAgreementJune302025UntilDecember312026Member_019fc957-9c66-7bbe-9464-a073703bcdbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementBeginning2024Member_019fc95c-99ab-7649-bb4e-8eba89a33a03" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementBeginning2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:to="loc_itg_PreviousCreditAgreementBeginning2024Member_019fc95c-99ab-7649-bb4e-8eba89a33a03" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementBeginningWithTheQuarterEndedJune302025UntilSeptember302026Member_019fc95d-61c0-7a8e-aa9e-79525c70121f" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementBeginningWithTheQuarterEndedJune302025UntilSeptember302026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:to="loc_itg_PreviousCreditAgreementBeginningWithTheQuarterEndedJune302025UntilSeptember302026Member_019fc95d-61c0-7a8e-aa9e-79525c70121f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PreviousCreditAgreementJune302025UntilSeptember302026Member_019fc95e-9ed9-70b3-b975-c678bafdb9a3" xlink:href="itg-20260630.xsd#itg_PreviousCreditAgreementJune302025UntilSeptember302026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_itg_PreviousCreditAgreementMember_019fc955-e89f-7be4-9b15-962d9fd68a98" xlink:to="loc_itg_PreviousCreditAgreementJune302025UntilSeptember302026Member_019fc95e-9ed9-70b3-b975-c678bafdb9a3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_EquipmentLoansMember_019fc964-c973-750e-8582-6ac94c5a14bb" xlink:href="itg-20260630.xsd#itg_EquipmentLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc937-35a3-7985-8b4f-2538c3603622" xlink:to="loc_itg_EquipmentLoansMember_019fc964-c973-750e-8582-6ac94c5a14bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019fc937-35a3-7a96-87de-9c70d94e9360" 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_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:to="loc_us-gaap_VariableRateAxis_019fc937-35a3-7a96-87de-9c70d94e9360" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019fc937-35a3-7a96-87de-9c70d94e9360_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_019fc937-35a3-7a96-87de-9c70d94e9360" xlink:to="loc_us-gaap_VariableRateDomain_019fc937-35a3-7a96-87de-9c70d94e9360_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019fc937-35a3-702f-9dc4-0a68a667991c" 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_019fc937-35a3-7a96-87de-9c70d94e9360" xlink:to="loc_us-gaap_VariableRateDomain_019fc937-35a3-702f-9dc4-0a68a667991c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PrimeRateOrFederalFundsRateMember_019fc947-abf8-7292-a575-0da5a4b1021a" xlink:href="itg-20260630.xsd#itg_PrimeRateOrFederalFundsRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019fc937-35a3-702f-9dc4-0a68a667991c" xlink:to="loc_itg_PrimeRateOrFederalFundsRateMember_019fc947-abf8-7292-a575-0da5a4b1021a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019fc948-03a1-7c90-ae26-e797e7175720" 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_019fc937-35a3-702f-9dc4-0a68a667991c" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019fc948-03a1-7c90-ae26-e797e7175720" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019fc94a-3b4b-75f1-aa2e-0d17d82ddcac" 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_019fc937-35a3-702f-9dc4-0a68a667991c" xlink:to="loc_us-gaap_BaseRateMember_019fc94a-3b4b-75f1-aa2e-0d17d82ddcac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019fc938-31f3-7296-ab25-89d799d14c23" 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_DebtInstrumentTable_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:to="loc_us-gaap_CreditFacilityAxis_019fc938-31f3-7296-ab25-89d799d14c23" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc938-31f3-7296-ab25-89d799d14c23_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_019fc938-31f3-7296-ab25-89d799d14c23" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc938-31f3-7296-ab25-89d799d14c23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc938-31f3-71ab-bae3-35bccb10427b" 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_019fc938-31f3-7296-ab25-89d799d14c23" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc938-31f3-71ab-bae3-35bccb10427b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019fc938-9505-7014-acac-0caeafa5d52d" 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_CreditFacilityDomain_019fc938-31f3-71ab-bae3-35bccb10427b" xlink:to="loc_us-gaap_SecuredDebtMember_019fc938-9505-7014-acac-0caeafa5d52d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019fc938-b28a-7042-a4b6-8cdffcda3e7d" 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_019fc938-31f3-71ab-bae3-35bccb10427b" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019fc938-b28a-7042-a4b6-8cdffcda3e7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019fc963-6620-7b41-bf52-f762093d6266" 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_019fc937-35a3-7c8b-b9b8-be9b5ef4c0b7" xlink:to="loc_srt_RangeAxis_019fc963-6620-7b41-bf52-f762093d6266" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fc963-6620-7b41-bf52-f762093d6266_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_019fc963-6620-7b41-bf52-f762093d6266" xlink:to="loc_srt_RangeMember_019fc963-6620-7b41-bf52-f762093d6266_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fc963-6620-71aa-9fe4-5ef7f49afd36" 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_019fc963-6620-7b41-bf52-f762093d6266" xlink:to="loc_srt_RangeMember_019fc963-6620-71aa-9fe4-5ef7f49afd36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019fc963-7f9f-7a5c-8353-e7280813792d" 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_019fc963-6620-71aa-9fe4-5ef7f49afd36" xlink:to="loc_srt_MinimumMember_019fc963-7f9f-7a5c-8353-e7280813792d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019fc963-9655-72d1-95ab-b9cc747a8eaa" 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_019fc963-6620-71aa-9fe4-5ef7f49afd36" xlink:to="loc_srt_MaximumMember_019fc963-9655-72d1-95ab-b9cc747a8eaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc937-35a3-787c-aa96-7c6f9d984c2c" 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_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc937-35a3-787c-aa96-7c6f9d984c2c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019fc937-8d1b-784c-84cd-866e6075a325" 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_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019fc937-8d1b-784c-84cd-866e6075a325" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019fc93a-b2fb-787a-b947-e91ee216373d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentPeriodicPaymentPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019fc93a-b2fb-787a-b947-e91ee216373d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_DebtInstrumentPeriodicPaymentPrincipalPercentage_019fc93c-0843-725a-9b36-747a943979fa" xlink:href="itg-20260630.xsd#itg_DebtInstrumentPeriodicPaymentPrincipalPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_itg_DebtInstrumentPeriodicPaymentPrincipalPercentage_019fc93c-0843-725a-9b36-747a943979fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019fc93e-7b89-7499-8f01-6ac0e4630ad5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019fc93e-7b89-7499-8f01-6ac0e4630ad5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019fc945-4ed0-78b0-b4be-aec376e4d035" 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_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019fc945-4ed0-78b0-b4be-aec376e4d035" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_DebtInstrumentApplicableVariableRate_019fc949-6b37-739c-b67f-754ceb51714a" xlink:href="itg-20260630.xsd#itg_DebtInstrumentApplicableVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_itg_DebtInstrumentApplicableVariableRate_019fc949-6b37-739c-b67f-754ceb51714a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_019fc94b-4103-74e3-951c-3a478406da70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_019fc94b-4103-74e3-951c-3a478406da70" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_LineOfCreditFacilityMandatoryPrepaymentPercentageOfExcessCashFlow_019fc950-585b-7b4e-81d4-6a1cf0b7acc4" xlink:href="itg-20260630.xsd#itg_LineOfCreditFacilityMandatoryPrepaymentPercentageOfExcessCashFlow"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_itg_LineOfCreditFacilityMandatoryPrepaymentPercentageOfExcessCashFlow_019fc950-585b-7b4e-81d4-6a1cf0b7acc4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_LineOfCreditFacilityMandatoryPrepaymentDueDatePeriodFromDeliveringAuditedFinancialStatements_019fc954-31b1-76f1-a2f8-23bb2f2b24c0" xlink:href="itg-20260630.xsd#itg_LineOfCreditFacilityMandatoryPrepaymentDueDatePeriodFromDeliveringAuditedFinancialStatements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_itg_LineOfCreditFacilityMandatoryPrepaymentDueDatePeriodFromDeliveringAuditedFinancialStatements_019fc954-31b1-76f1-a2f8-23bb2f2b24c0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019fc958-beee-7106-adfa-cef28606ccfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019fc958-beee-7106-adfa-cef28606ccfe" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fc962-c7b5-7eb5-83c2-060ad67f36bd" 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_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fc962-c7b5-7eb5-83c2-060ad67f36bd" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019fc964-2a11-77c1-ae3b-008a249e09a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc937-35a3-7dd7-b0ab-37bf43e93a63" xlink:to="loc_us-gaap_LongTermDebt_019fc964-2a11-77c1-ae3b-008a249e09a5" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/MEMBERSEQUITYDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#MEMBERSEQUITYDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/MEMBERSEQUITYDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionsMadeToLimitedLiabilityCompanyLLCMemberTable_019fc96a-e60b-778a-9f27-3a460d8e2ae4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DistributionsMadeToLimitedLiabilityCompanyLLCMemberTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_us-gaap_DistributionsMadeToLimitedLiabilityCompanyLLCMemberTable_019fc96a-e60b-778a-9f27-3a460d8e2ae4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fc96b-ba80-718b-85a3-b12c3bd3841c" 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_DistributionsMadeToLimitedLiabilityCompanyLLCMemberTable_019fc96a-e60b-778a-9f27-3a460d8e2ae4" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fc96b-ba80-718b-85a3-b12c3bd3841c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-718b-85a3-b12c3bd3841c_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_019fc96b-ba80-718b-85a3-b12c3bd3841c" xlink:to="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-718b-85a3-b12c3bd3841c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" 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_019fc96b-ba80-718b-85a3-b12c3bd3841c" xlink:to="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassAUnitMember_019fc96c-0be0-7852-8db3-6c7e4e360160" xlink:href="itg-20260630.xsd#itg_ClassAUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" xlink:to="loc_itg_ClassAUnitMember_019fc96c-0be0-7852-8db3-6c7e4e360160" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassBUnitMember_019fc96e-70f3-7614-b02e-443ed585c637" xlink:href="itg-20260630.xsd#itg_ClassBUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" xlink:to="loc_itg_ClassBUnitMember_019fc96e-70f3-7614-b02e-443ed585c637" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassCUnitMember_019fc970-b41e-79c1-9915-4498f6a1b6b0" xlink:href="itg-20260630.xsd#itg_ClassCUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" xlink:to="loc_itg_ClassCUnitMember_019fc970-b41e-79c1-9915-4498f6a1b6b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassXUnitMember_019fc972-84fd-712b-a85a-b34494240ff0" xlink:href="itg-20260630.xsd#itg_ClassXUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019fc96b-ba80-7302-8cf1-dc2ea41fe06c" xlink:to="loc_itg_ClassXUnitMember_019fc972-84fd-712b-a85a-b34494240ff0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019fc971-a85b-7d16-83d3-fbcf37719ab3" 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_DistributionsMadeToLimitedLiabilityCompanyLLCMemberTable_019fc96a-e60b-778a-9f27-3a460d8e2ae4" xlink:to="loc_us-gaap_AwardTypeAxis_019fc971-a85b-7d16-83d3-fbcf37719ab3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc971-a85b-7d16-83d3-fbcf37719ab3_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_019fc971-a85b-7d16-83d3-fbcf37719ab3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc971-a85b-7d16-83d3-fbcf37719ab3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc971-a85b-7582-ad7a-caf28383f272" 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_019fc971-a85b-7d16-83d3-fbcf37719ab3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc971-a85b-7582-ad7a-caf28383f272" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassDUnitMember_019fc971-be19-77fa-9f59-c85f18fc09b5" xlink:href="itg-20260630.xsd#itg_ClassDUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc971-a85b-7582-ad7a-caf28383f272" xlink:to="loc_itg_ClassDUnitMember_019fc971-be19-77fa-9f59-c85f18fc09b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonUnitOutstanding_019fc96a-e60b-764a-8713-2b5543ea1f51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonUnitOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_us-gaap_CommonUnitOutstanding_019fc96a-e60b-764a-8713-2b5543ea1f51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CommonUnitVested_019fc96d-8a03-7852-88e2-ebb41e2bce67" xlink:href="itg-20260630.xsd#itg_CommonUnitVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_itg_CommonUnitVested_019fc96d-8a03-7852-88e2-ebb41e2bce67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CommonUnitUnvested_019fc96d-dbb9-74dd-846e-0a4e2b97471e" xlink:href="itg-20260630.xsd#itg_CommonUnitUnvested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_itg_CommonUnitUnvested_019fc96d-dbb9-74dd-846e-0a4e2b97471e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fc974-bb26-70b5-bb17-9d216078c78e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fc974-bb26-70b5-bb17-9d216078c78e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_DistributionMadeToLimitedLiabilityCompanyLLCMemberMaximumAmountReceivable_019fc976-aef1-7834-b9b1-906e42c55107" xlink:href="itg-20260630.xsd#itg_DistributionMadeToLimitedLiabilityCompanyLLCMemberMaximumAmountReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_itg_DistributionMadeToLimitedLiabilityCompanyLLCMemberMaximumAmountReceivable_019fc976-aef1-7834-b9b1-906e42c55107" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019fc971-8d1c-75f4-8981-1858699b3096" 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_DistributionMadeToLimitedLiabilityCompanyLLCMemberLineItems_019fc96a-e60b-7822-b084-d85c385149a8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019fc971-8d1c-75f4-8981-1858699b3096" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/EQUITYBASEDCOMPENSATIONDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#EQUITYBASEDCOMPENSATIONDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/EQUITYBASEDCOMPENSATIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" 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_019fc514-25df-7b48-b0c7-a73d5e854897" 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_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019fc514-25df-7b48-b0c7-a73d5e854897" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019fc518-68f4-7caa-8d59-b4cda1efdb02" 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_019fc514-25df-7b48-b0c7-a73d5e854897" xlink:to="loc_us-gaap_AwardTypeAxis_019fc518-68f4-7caa-8d59-b4cda1efdb02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc518-68f4-7caa-8d59-b4cda1efdb02_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_019fc518-68f4-7caa-8d59-b4cda1efdb02" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc518-68f4-7caa-8d59-b4cda1efdb02_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc518-68f4-7c0d-9dfe-a55cdf0d7cd6" 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_019fc518-68f4-7caa-8d59-b4cda1efdb02" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc518-68f4-7c0d-9dfe-a55cdf0d7cd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassDUnitMember_019fc518-d47a-7e67-956f-8e9fa8040e2b" xlink:href="itg-20260630.xsd#itg_ClassDUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fc518-68f4-7c0d-9dfe-a55cdf0d7cd6" xlink:to="loc_itg_ClassDUnitMember_019fc518-d47a-7e67-956f-8e9fa8040e2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019fc514-25df-79c8-b5f1-1349ff8c85d1" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019fc514-25df-79c8-b5f1-1349ff8c85d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019fc514-8a4b-7c88-a73c-483882cc9e0e" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019fc514-8a4b-7c88-a73c-483882cc9e0e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodGrantDateFairValue_019fc517-3ea2-795b-abed-b12cd08c71f2" xlink:href="itg-20260630.xsd#itg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_itg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodGrantDateFairValue_019fc517-3ea2-795b-abed-b12cd08c71f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019fc514-d62e-7107-a0b4-631da4621b9b" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019fc514-d62e-7107-a0b4-631da4621b9b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019fc515-f2b2-7e02-beeb-48a5a5561db3" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019fc515-f2b2-7e02-beeb-48a5a5561db3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019fc516-1a6e-738d-8795-84663dc11eed" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019fc514-25df-7099-b1a5-3fc9515d5f32" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019fc516-1a6e-738d-8795-84663dc11eed" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/SEGMENTREPORTINGScheduleofOperatingSegmentFinancialInformationDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#SEGMENTREPORTINGScheduleofOperatingSegmentFinancialInformationDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/SEGMENTREPORTINGScheduleofOperatingSegmentFinancialInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" 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_019fbf81-ca33-7470-873d-1aaee031d138" 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_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019fbf81-ca33-7470-873d-1aaee031d138" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019fbf81-e88a-7ce6-8931-54d963fa6274" 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_019fbf81-ca33-7470-873d-1aaee031d138" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019fbf81-e88a-7ce6-8931-54d963fa6274" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fbf81-e88a-7ce6-8931-54d963fa6274_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_019fbf81-e88a-7ce6-8931-54d963fa6274" xlink:to="loc_us-gaap_SegmentDomain_019fbf81-e88a-7ce6-8931-54d963fa6274_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fbf81-e88b-7499-ab60-6f4fd293cffe" 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_019fbf81-e88a-7ce6-8931-54d963fa6274" xlink:to="loc_us-gaap_SegmentDomain_019fbf81-e88b-7499-ab60-6f4fd293cffe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ReportableSegmentMember_019fbf82-4ad3-78c7-b348-2876455633a0" xlink:href="itg-20260630.xsd#itg_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019fbf81-e88b-7499-ab60-6f4fd293cffe" xlink:to="loc_itg_ReportableSegmentMember_019fbf82-4ad3-78c7-b348-2876455633a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019fbf81-ca33-7267-88a8-1d320840b3c5" 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_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019fbf81-ca33-7267-88a8-1d320840b3c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019fbf81-ca33-7e8d-8b84-0dcc7f114d5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019fbf81-ca33-7e8d-8b84-0dcc7f114d5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019fbf81-ca33-7711-818b-e4ca7c9a022b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019fbf81-ca33-7711-818b-e4ca7c9a022b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019fbf81-ca33-7807-bbe3-12e89eb01fa2" 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_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019fbf81-ca33-7807-bbe3-12e89eb01fa2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_019fbf81-ca33-72c5-9846-78d4b6e5385b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_019fbf81-ca33-72c5-9846-78d4b6e5385b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_019fbf81-ca33-7601-b4bc-b8b627de42a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_CostsAndExpenses_019fbf81-ca33-7601-b4bc-b8b627de42a1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019fbf81-ca33-7845-93f8-55afb98fb5ad" 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_SegmentReportingInformationLineItems_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019fbf81-ca33-7845-93f8-55afb98fb5ad" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019fbf81-ca33-71e0-b931-d1dbe0bced57" 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_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019fbf81-ca33-71e0-b931-d1dbe0bced57" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019fbf81-ca33-78f2-9c8d-10d833e76cd9" 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_019fbf81-ca33-79ce-ba39-071cbbe45f2c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019fbf81-ca33-78f2-9c8d-10d833e76cd9" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/MAJORCUSTOMERSDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#MAJORCUSTOMERSDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/MAJORCUSTOMERSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019fc3ee-d092-7af3-8565-ca1fceb6eaff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019fc3ee-d092-78af-914a-362de09ab09b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fc3ee-d092-7af3-8565-ca1fceb6eaff" xlink:to="loc_us-gaap_ConcentrationRiskTable_019fc3ee-d092-78af-914a-362de09ab09b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fc3ee-d092-78af-914a-362de09ab09b" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-79c2-a707-60de2407320e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019fc3ef-19cf-7745-a5b7-2a17d37e0bf2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-79c2-a707-60de2407320e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019fc3ef-c880-717d-be34-9b6ce0b5ea18" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-79c2-a707-60de2407320e" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019fc3ef-c880-717d-be34-9b6ce0b5ea18" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019fc3f0-c136-7105-8c0c-0d061284aa0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019fc3ef-19cf-79c2-a707-60de2407320e" xlink:to="loc_us-gaap_AccountsReceivableMember_019fc3f0-c136-7105-8c0c-0d061284aa0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019fc3ef-311d-7c75-b295-50d38f358821" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fc3ee-d092-78af-914a-362de09ab09b" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019fc3ef-311d-7c75-b295-50d38f358821" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019fc3ef-311d-7c75-b295-50d38f358821_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019fc3ef-311d-7c75-b295-50d38f358821" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019fc3ef-311d-7c75-b295-50d38f358821_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019fc3ef-311d-7148-9874-f8c86f869ef9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019fc3ef-311d-7c75-b295-50d38f358821" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019fc3ef-311d-7148-9874-f8c86f869ef9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019fc3ef-8613-750f-b8ae-171e55650876" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019fc3ef-311d-7148-9874-f8c86f869ef9" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019fc3ef-8613-750f-b8ae-171e55650876" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019fc3ef-45a8-7502-aee3-30566bf7ae21" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fc3ee-d092-78af-914a-362de09ab09b" xlink:to="loc_srt_MajorCustomersAxis_019fc3ef-45a8-7502-aee3-30566bf7ae21" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7502-aee3-30566bf7ae21_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019fc3ef-45a8-7502-aee3-30566bf7ae21" xlink:to="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7502-aee3-30566bf7ae21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7680-82e2-fe594b534c39" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019fc3ef-45a8-7502-aee3-30566bf7ae21" xlink:to="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7680-82e2-fe594b534c39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CustomerOneMember_019fc3f1-0263-76bb-a856-9b8353c53109" xlink:href="itg-20260630.xsd#itg_CustomerOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7680-82e2-fe594b534c39" xlink:to="loc_itg_CustomerOneMember_019fc3f1-0263-76bb-a856-9b8353c53109" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CustomerTwoMember_019fc3f1-6421-7421-b35f-5cbc5fab58f2" xlink:href="itg-20260630.xsd#itg_CustomerTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019fc3ef-45a8-7680-82e2-fe594b534c39" xlink:to="loc_itg_CustomerTwoMember_019fc3f1-6421-7421-b35f-5cbc5fab58f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019fc3ee-d092-7946-a057-381d74d9df47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fc3ee-d092-7af3-8565-ca1fceb6eaff" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019fc3ee-d092-7946-a057-381d74d9df47" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/RELATEDPARTYTRANSACTIONSDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#RELATEDPARTYTRANSACTIONSDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/RELATEDPARTYTRANSACTIONSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019fc98a-a6df-7298-b69e-032174ffdba0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019fc98a-a6df-7298-b69e-032174ffdba0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019fc98b-3bf6-7f1a-8ce3-5860557d9df1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019fc98a-a6df-7298-b69e-032174ffdba0" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019fc98b-3bf6-7f1a-8ce3-5860557d9df1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7f1a-8ce3-5860557d9df1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019fc98b-3bf6-7f1a-8ce3-5860557d9df1" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7f1a-8ce3-5860557d9df1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019fc98b-3bf6-7f1a-8ce3-5860557d9df1" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_CertainServicesMember_019fc98b-d6fa-7226-92fa-3fbb6f472904" xlink:href="itg-20260630.xsd#itg_CertainServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:to="loc_itg_CertainServicesMember_019fc98b-d6fa-7226-92fa-3fbb6f472904" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_SubcontractorServicesMember_019fc98c-0772-7ba1-8f81-10a4bc560e31" xlink:href="itg-20260630.xsd#itg_SubcontractorServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:to="loc_itg_SubcontractorServicesMember_019fc98c-0772-7ba1-8f81-10a4bc560e31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PurchaseOfUniformAndApparelProductsMember_019fc98c-4ebf-7cea-b8c3-976dd9007749" xlink:href="itg-20260630.xsd#itg_PurchaseOfUniformAndApparelProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:to="loc_itg_PurchaseOfUniformAndApparelProductsMember_019fc98c-4ebf-7cea-b8c3-976dd9007749" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_NoteReceivableFromBusinessAcquisitionMember_019fdd00-907c-72d1-80d9-9d6c9d77b5ad" xlink:href="itg-20260630.xsd#itg_NoteReceivableFromBusinessAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019fc98b-3bf6-7a07-9580-fc4c81c59e08" xlink:to="loc_itg_NoteReceivableFromBusinessAcquisitionMember_019fdd00-907c-72d1-80d9-9d6c9d77b5ad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019fc98e-aeed-71c2-8109-5cbb8a7aee55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019fc98a-a6df-7298-b69e-032174ffdba0" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019fc98e-aeed-71c2-8109-5cbb8a7aee55" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019fc98e-aeed-71c2-8109-5cbb8a7aee55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019fc98e-aeed-71c2-8109-5cbb8a7aee55" xlink:to="loc_us-gaap_RelatedPartyDomain_019fc98e-aeed-71c2-8109-5cbb8a7aee55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019fc98e-aeed-79ba-acd9-4f85ffa7062f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019fc98e-aeed-71c2-8109-5cbb8a7aee55" xlink:to="loc_us-gaap_RelatedPartyDomain_019fc98e-aeed-79ba-acd9-4f85ffa7062f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AffiliatedEntityMember_019fc98e-ec09-723b-a0c2-d15aa8f0e584" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_AffiliatedEntityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019fc98e-aeed-79ba-acd9-4f85ffa7062f" xlink:to="loc_srt_AffiliatedEntityMember_019fc98e-ec09-723b-a0c2-d15aa8f0e584" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a" 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_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019fc98a-a6df-7298-b69e-032174ffdba0" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a_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_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc9a6-727c-71f5-8b52-0d65d971f7ef" 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_019fc9a6-727c-7d80-92ea-0feb1a1a5e9a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc9a6-727c-71f5-8b52-0d65d971f7ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ITGOpcoMember_019fc9a6-ed3c-71e5-8afa-b16451bcde9b" xlink:href="itg-20260630.xsd#itg_ITGOpcoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc9a6-727c-71f5-8b52-0d65d971f7ef" xlink:to="loc_itg_ITGOpcoMember_019fc9a6-ed3c-71e5-8afa-b16451bcde9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_019fc98a-a6df-7901-9d9c-e1335e23e07d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionAmountsOfTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_019fc98a-a6df-7901-9d9c-e1335e23e07d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_019fc98e-324d-7d53-bb4b-64009fb7256d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_OperatingLeasePayments_019fc98e-324d-7d53-bb4b-64009fb7256d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionRate_019fdcfe-69e2-702e-b6ec-b850d83deea9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_RelatedPartyTransactionRate_019fdcfe-69e2-702e-b6ec-b850d83deea9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestReceivable_019fdd01-f156-700c-a44c-1e61e27b3ee4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_InterestReceivable_019fdd01-f156-700c-a44c-1e61e27b3ee4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_019fc9a3-6180-7fb8-8fae-e28aa4003899" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_019fc9a3-6180-7fb8-8fae-e28aa4003899" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_PaymentForContingentConsiderationLiability_019ff1c6-b02c-7e40-886c-b32e7cc69e2f" xlink:href="itg-20260630.xsd#itg_PaymentForContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_itg_PaymentForContingentConsiderationLiability_019ff1c6-b02c-7e40-886c-b32e7cc69e2f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_BusinessCombinationContingentConsiderationInterestRate_019fc9a7-b67a-7518-b9a3-b6822e3d2e48" xlink:href="itg-20260630.xsd#itg_BusinessCombinationContingentConsiderationInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_itg_BusinessCombinationContingentConsiderationInterestRate_019fc9a7-b67a-7518-b9a3-b6822e3d2e48" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_019fc9a2-b496-7a0f-b4ca-e6b8aa35c50d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019fc98a-a6df-7e61-8b33-de9d4fc2c41c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_019fc9a2-b496-7a0f-b4ca-e6b8aa35c50d" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://prod.itgcomm.com/role/SUBSEQUENTEVENTSDetails" xlink:type="simple" xlink:href="itg-20260630.xsd#SUBSEQUENTEVENTSDetails"/>
  <link:definitionLink xlink:role="http://prod.itgcomm.com/role/SUBSEQUENTEVENTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fce5b-177c-75ee-8fee-01700cc7cd6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fce5b-177c-75ee-8fee-01700cc7cd6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fce5b-177c-75ee-8fee-01700cc7cd6c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fce5b-177c-75ee-8fee-01700cc7cd6c" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fce5b-177c-75ee-8fee-01700cc7cd6c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fce5b-177c-79b5-ad14-bbcbaac03966" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fce5b-177c-75ee-8fee-01700cc7cd6c" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fce5b-177c-79b5-ad14-bbcbaac03966" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fce5b-177c-73a8-910c-7937df7d5a94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fce5b-177c-79b5-ad14-bbcbaac03966" xlink:to="loc_us-gaap_SubsequentEventMember_019fce5b-177c-73a8-910c-7937df7d5a94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce5b-177c-740a-a530-de54c0486260" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce5b-177c-740a-a530-de54c0486260" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-740a-a530-de54c0486260_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce5b-177c-740a-a530-de54c0486260" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-740a-a530-de54c0486260_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-7ced-b00a-6b82709b4521" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019fce5b-177c-740a-a530-de54c0486260" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-7ced-b00a-6b82709b4521" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_019fce5b-177c-75ff-b27e-9cd28f406ceb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-7ced-b00a-6b82709b4521" xlink:to="loc_us-gaap_IPOMember_019fce5b-177c-75ff-b27e-9cd28f406ceb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019fce94-980d-7425-9948-d8ec98cd7138" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019fce5b-177c-7ced-b00a-6b82709b4521" xlink:to="loc_us-gaap_PrivatePlacementMember_019fce94-980d-7425-9948-d8ec98cd7138" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_019fce5b-6e1d-7b71-903b-e5b39c7c949f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_srt_OwnershipAxis_019fce5b-6e1d-7b71-903b-e5b39c7c949f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fce5b-6e1d-7b71-903b-e5b39c7c949f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_019fce5b-6e1d-7b71-903b-e5b39c7c949f" xlink:to="loc_srt_OwnershipDomain_019fce5b-6e1d-7b71-903b-e5b39c7c949f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019fce5b-6e1d-7a74-973f-77a1466a0c2e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_019fce5b-6e1d-7b71-903b-e5b39c7c949f" xlink:to="loc_srt_OwnershipDomain_019fce5b-6e1d-7a74-973f-77a1466a0c2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ITGParentMember_019fce5b-6e1d-7743-b5be-eaa6a4aa3e98" xlink:href="itg-20260630.xsd#itg_ITGParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_019fce5b-6e1d-7a74-973f-77a1466a0c2e" xlink:to="loc_itg_ITGParentMember_019fce5b-6e1d-7743-b5be-eaa6a4aa3e98" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019fce5c-b23c-7405-8570-7f8265702b26" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_dei_LegalEntityAxis_019fce5c-b23c-7405-8570-7f8265702b26" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fce5c-b23c-7405-8570-7f8265702b26_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019fce5c-b23c-7405-8570-7f8265702b26" xlink:to="loc_dei_EntityDomain_019fce5c-b23c-7405-8570-7f8265702b26_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fce5c-b23c-75c4-8740-ab820e754bf3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019fce5c-b23c-7405-8570-7f8265702b26" xlink:to="loc_dei_EntityDomain_019fce5c-b23c-75c4-8740-ab820e754bf3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ITGInc.Member_019fce5c-e796-70ec-bf60-d802348b835e" xlink:href="itg-20260630.xsd#itg_ITGInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019fce5c-b23c-75c4-8740-ab820e754bf3" xlink:to="loc_itg_ITGInc.Member_019fce5c-e796-70ec-bf60-d802348b835e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fce94-980d-7576-8709-d382ad7edb21" 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_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fce94-980d-7576-8709-d382ad7edb21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fce94-980d-7576-8709-d382ad7edb21_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_019fce94-980d-7576-8709-d382ad7edb21" xlink:to="loc_us-gaap_ClassOfStockDomain_019fce94-980d-7576-8709-d382ad7edb21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fce94-980d-7210-a0b7-1bfee41da084" 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_019fce94-980d-7576-8709-d382ad7edb21" xlink:to="loc_us-gaap_ClassOfStockDomain_019fce94-980d-7210-a0b7-1bfee41da084" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019fce94-980d-7675-a0de-67dd8c989d08" 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_019fce94-980d-7210-a0b7-1bfee41da084" xlink:to="loc_us-gaap_CommonClassBMember_019fce94-980d-7675-a0de-67dd8c989d08" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019fce96-1f19-76d5-b6e0-991a8267e472" 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_019fce94-980d-7210-a0b7-1bfee41da084" xlink:to="loc_us-gaap_CommonClassAMember_019fce96-1f19-76d5-b6e0-991a8267e472" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fce96-1f19-7390-b6b9-f2b5d0d39dbb" 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_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fce96-1f19-7390-b6b9-f2b5d0d39dbb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce96-1f19-7390-b6b9-f2b5d0d39dbb_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_019fce96-1f19-7390-b6b9-f2b5d0d39dbb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce96-1f19-7390-b6b9-f2b5d0d39dbb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce96-1f19-742d-b9b5-a19332024cdd" 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_019fce96-1f19-7390-b6b9-f2b5d0d39dbb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce96-1f19-742d-b9b5-a19332024cdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_OaktreeBlockedFundMember_019fce96-1f19-74c8-82a3-9475a8913f58" xlink:href="itg-20260630.xsd#itg_OaktreeBlockedFundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fce96-1f19-742d-b9b5-a19332024cdd" xlink:to="loc_itg_OaktreeBlockedFundMember_019fce96-1f19-74c8-82a3-9475a8913f58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019fe920-9d23-71dc-8628-886c978989cc" 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_SubsequentEventTable_019fce5b-177c-78e2-907b-e5d7cb37365e" xlink:to="loc_us-gaap_AwardTypeAxis_019fe920-9d23-71dc-8628-886c978989cc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fe920-9d23-71dc-8628-886c978989cc_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_019fe920-9d23-71dc-8628-886c978989cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fe920-9d23-71dc-8628-886c978989cc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fe920-9d23-7d5b-af86-21be8a863096" 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_019fe920-9d23-71dc-8628-886c978989cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fe920-9d23-7d5b-af86-21be8a863096" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_ClassDUnitMember_019fe920-bc0a-7a6e-a9e2-e87d040aafc4" xlink:href="itg-20260630.xsd#itg_ClassDUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019fe920-9d23-7d5b-af86-21be8a863096" xlink:to="loc_itg_ClassDUnitMember_019fe920-bc0a-7a6e-a9e2-e87d040aafc4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fce5b-177c-7432-80ca-068ab15c5fa0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fce5b-177c-7432-80ca-068ab15c5fa0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019fce5b-3565-7047-9487-edd64c27229c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019fce5b-3565-7047-9487-edd64c27229c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019fce5b-5012-7a81-b348-587f34e99ce8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019fce5b-5012-7a81-b348-587f34e99ce8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalanceShares_019fce5b-6e1d-752f-a85b-3bb4746ff00b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentOwnedBalanceShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_InvestmentOwnedBalanceShares_019fce5b-6e1d-752f-a85b-3bb4746ff00b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fce5b-88ba-756e-b299-9ae736ca07a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_019fce5b-88ba-756e-b299-9ae736ca07a2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019fe91f-e6c5-7c35-bf7c-ebc159d6d79a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019fe91f-e6c5-7c35-bf7c-ebc159d6d79a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fce94-980d-7c5f-8ec8-9891f10309ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019fce94-980d-7c5f-8ec8-9891f10309ff" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019fce96-1f19-7a32-8fd5-2d2dfb958f06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019fce96-1f19-7a32-8fd5-2d2dfb958f06" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_itg_RequiredRatioOfCommonStockToInvestmentOwnedShares_019fce9a-9d35-77ba-834e-5e23f7c39405" xlink:href="itg-20260630.xsd#itg_RequiredRatioOfCommonStockToInvestmentOwnedShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fce5b-177c-73bd-8e5b-7754fa7d3ed3" xlink:to="loc_itg_RequiredRatioOfCommonStockToInvestmentOwnedShares_019fce9a-9d35-77ba-834e-5e23f7c39405" xlink:type="arc" order="8"/>
  </link:definitionLink>
</link:linkbase>
