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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a010e8-d5af-7c47-ae37-fee6e89151a1,g:5eb166e4-7229-4a34-afe4-e6a48d4ce0f7-->
<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://www2.allegiantair.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYunaudited" xlink:type="simple" xlink:href="algt-20260930.xsd#CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYunaudited"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYunaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a010e8-d6ce-7676-8ae1-6137eef39303" 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_01a010e8-d6ce-7de8-b816-6b5d43be778e" 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_01a010e8-d6ce-7676-8ae1-6137eef39303" xlink:to="loc_us-gaap_StatementTable_01a010e8-d6ce-7de8-b816-6b5d43be778e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a010e8-d6ce-75cb-a62b-70532b7e0321" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_01a010e8-d6ce-7de8-b816-6b5d43be778e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a010e8-d6ce-75cb-a62b-70532b7e0321" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-75cb-a62b-70532b7e0321_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a010e8-d6ce-75cb-a62b-70532b7e0321" xlink:to="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-75cb-a62b-70532b7e0321_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a010e8-d6ce-75cb-a62b-70532b7e0321" xlink:to="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a010e8-d6ce-7e87-96aa-1be775d90c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:to="loc_us-gaap_CommonStockMember_01a010e8-d6ce-7e87-96aa-1be775d90c54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a010e8-d6ce-7963-a893-f8feaf538280" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a010e8-d6ce-7963-a893-f8feaf538280" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a010e8-d6ce-7149-8bda-aa27cecc0f86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a010e8-d6ce-7149-8bda-aa27cecc0f86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_01a010e8-d6ce-79dc-b36d-80c52849fc83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:to="loc_us-gaap_RetainedEarningsMember_01a010e8-d6ce-79dc-b36d-80c52849fc83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_01a010e8-d6ce-75e2-821a-0b60d8f3ba6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a010e8-d6ce-719c-9cb0-1077be853d8b" xlink:to="loc_us-gaap_TreasuryStockCommonMember_01a010e8-d6ce-75e2-821a-0b60d8f3ba6c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a010e8-d6ce-7676-8ae1-6137eef39303" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a010e8-d6ce-7d53-8be5-50d89f05d35c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_01a010e8-d6ce-7d53-8be5-50d89f05d35c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a010e8-d6ce-7da5-98e6-87b8fd6f6a3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_StockholdersEquity_01a010e8-d6ce-7da5-98e6-87b8fd6f6a3a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a010e8-d6ce-7b71-a910-fb43831e092a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a010e8-d6ce-7b71-a910-fb43831e092a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a010e8-d6ce-7d19-a253-61d93877f88b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a010e8-d6ce-7d19-a253-61d93877f88b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_01a010e8-d6ce-763d-b781-d625e1be2174" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_01a010e8-d6ce-763d-b781-d625e1be2174" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_01a010e8-d6ce-7efa-b9a4-d50d936d0b47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_01a010e8-d6ce-7efa-b9a4-d50d936d0b47" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_01a010e8-d6ce-7cdb-b8a8-379063bc3a82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_01a010e8-d6ce-7cdb-b8a8-379063bc3a82" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_01a010e8-d6ce-7d45-9e71-d277ebc76545" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_01a010e8-d6ce-7d45-9e71-d277ebc76545" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_01a010e8-d6ce-7efc-8da4-9dc2daf82003" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_01a010e8-d6ce-7efc-8da4-9dc2daf82003" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_01a010e8-d6ce-73af-9a79-799dc0d8303f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_01a010e8-d6ce-73af-9a79-799dc0d8303f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_01a010e8-d6ce-7e90-aab2-167e6bcadd7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_01a010e8-d6ce-7e90-aab2-167e6bcadd7f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_01a010e8-d6ce-7b38-bdbb-ca624c19189e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a010e8-d6ce-706c-8ea4-21c371ab3ca7" xlink:to="loc_us-gaap_ProfitLoss_01a010e8-d6ce-7b38-bdbb-ca624c19189e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a010e8-d6ce-7dfc-92fc-8899b417ca3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a010e8-d6ce-7f85-a3c2-e0d0c2e82ca0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SpecialChargesNarrativeDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#SpecialChargesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/SpecialChargesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7462-82ed-36c21acd2660" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessInterruptionLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7421-ae93-79ed6149519d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7462-82ed-36c21acd2660" xlink:to="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7421-ae93-79ed6149519d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-7f72-a4c6-9d493aab388c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7421-ae93-79ed6149519d" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-7f72-a4c6-9d493aab388c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7f72-a4c6-9d493aab388c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-7f72-a4c6-9d493aab388c" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7f72-a4c6-9d493aab388c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7c66-84e2-15542e877b67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-7f72-a4c6-9d493aab388c" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7c66-84e2-15542e877b67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OrganizationalRestructuringMember_01a010e8-d6ce-7b2e-a34c-0e55ca15f60e" xlink:href="algt-20260930.xsd#algt_OrganizationalRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7c66-84e2-15542e877b67" xlink:to="loc_algt_OrganizationalRestructuringMember_01a010e8-d6ce-7b2e-a34c-0e55ca15f60e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SaleOfSunseekerResortMember_01a010e8-d6ce-720b-8248-5c9f8f843ee7" xlink:href="algt-20260930.xsd#algt_SaleOfSunseekerResortMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-7c66-84e2-15542e877b67" xlink:to="loc_algt_SaleOfSunseekerResortMember_01a010e8-d6ce-720b-8248-5c9f8f843ee7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfAircraftRetired_01a010e8-d6ce-7fcb-916a-88b5bd386d39" xlink:href="algt-20260930.xsd#algt_NumberOfAircraftRetired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7462-82ed-36c21acd2660" xlink:to="loc_algt_NumberOfAircraftRetired_01a010e8-d6ce-7fcb-916a-88b5bd386d39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_01a010e8-d6ce-72ab-b0cc-b38eed2daa10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7462-82ed-36c21acd2660" xlink:to="loc_us-gaap_RestructuringCharges_01a010e8-d6ce-72ab-b0cc-b38eed2daa10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfFlightSimulators_01a010e8-d6ce-7d35-97dd-eebb7e062f08" xlink:href="algt-20260930.xsd#algt_NumberOfFlightSimulators"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7462-82ed-36c21acd2660" xlink:to="loc_algt_NumberOfFlightSimulators_01a010e8-d6ce-7d35-97dd-eebb7e062f08" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SpecialChargesSummaryofSpecialChargesDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#SpecialChargesSummaryofSpecialChargesDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/SpecialChargesSummaryofSpecialChargesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7c6e-a74e-e790f34ddcb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessInterruptionLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7e1c-a511-aa5fdb2b01bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7c6e-a74e-e790f34ddcb9" xlink:to="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7e1c-a511-aa5fdb2b01bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-76b2-a614-1bebe7aec701" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7e1c-a511-aa5fdb2b01bc" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-76b2-a614-1bebe7aec701" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-76b2-a614-1bebe7aec701_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-76b2-a614-1bebe7aec701" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-76b2-a614-1bebe7aec701_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_01a010e8-d6ce-76b2-a614-1bebe7aec701" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember_01a010e8-d6ce-7a7a-81c5-55e6ce804649" xlink:href="algt-20260930.xsd#algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember_01a010e8-d6ce-7a7a-81c5-55e6ce804649" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember_01a010e8-d6ce-714f-b1f6-db3d880eb320" xlink:href="algt-20260930.xsd#algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember_01a010e8-d6ce-714f-b1f6-db3d880eb320" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_IntegrationCostsMember_01a010e8-d6ce-77b8-81c2-3b6cf045c0f7" xlink:href="algt-20260930.xsd#algt_IntegrationCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_IntegrationCostsMember_01a010e8-d6ce-77b8-81c2-3b6cf045c0f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OrganizationalRestructuringMember_01a010e8-d6ce-7106-bfa0-59e6ebb9df4b" xlink:href="algt-20260930.xsd#algt_OrganizationalRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_OrganizationalRestructuringMember_01a010e8-d6ce-7106-bfa0-59e6ebb9df4b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditLossOnNoteReceivableMember_01a010e8-d6ce-725f-b375-03eff44dc4af" xlink:href="algt-20260930.xsd#algt_CreditLossOnNoteReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_CreditLossOnNoteReceivableMember_01a010e8-d6ce-725f-b375-03eff44dc4af" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunseekerSpecialChargesMember_01a010e8-d6ce-776d-9470-33efe825b0f0" xlink:href="algt-20260930.xsd#algt_SunseekerSpecialChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_01a010e8-d6ce-789e-8602-8885c2df5f6b" xlink:to="loc_algt_SunseekerSpecialChargesMember_01a010e8-d6ce-776d-9470-33efe825b0f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c" 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_ScheduleOfBusinessInsuranceRecoveriesTable_01a010e8-d6ce-7e1c-a511-aa5fdb2b01bc" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c_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_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7372-9ba1-59976ee52e36" 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_01a010e8-d6ce-7079-ac4a-c5b9e3ed0d1c" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7372-9ba1-59976ee52e36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirlineMember_01a010e8-d6ce-7472-ae2e-d2426e85ccf9" xlink:href="algt-20260930.xsd#algt_AirlineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7372-9ba1-59976ee52e36" xlink:to="loc_algt_AirlineMember_01a010e8-d6ce-7472-ae2e-d2426e85ccf9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_01a010e8-d6ce-73cd-8a10-60f76fc21475" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7c6e-a74e-e790f34ddcb9" xlink:to="loc_us-gaap_RestructuringCharges_01a010e8-d6ce-73cd-8a10-60f76fc21475" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RestructuringChargesNetOfRecoveries_01a010e8-d6ce-7e3a-a30d-70e9914d55d0" xlink:href="algt-20260930.xsd#algt_RestructuringChargesNetOfRecoveries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7c6e-a74e-e790f34ddcb9" xlink:to="loc_algt_RestructuringChargesNetOfRecoveries_01a010e8-d6ce-7e3a-a30d-70e9914d55d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_01a010e8-d6ce-7014-8b61-f8ea003c75ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringSettlementAndImpairmentProvisions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_01a010e8-d6ce-7c6e-a74e-e790f34ddcb9" xlink:to="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_01a010e8-d6ce-7014-8b61-f8ea003c75ff" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/RevenueRecognitionDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#RevenueRecognitionDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/RevenueRecognitionDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_01a010e8-d6ce-7a29-abf7-55fb90200fcb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7112-af92-5d1bb039a954" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a010e8-d6ce-7a29-abf7-55fb90200fcb" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7112-af92-5d1bb039a954" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a010e8-d6ce-749a-9d60-486ebb2398d7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7112-af92-5d1bb039a954" xlink:to="loc_srt_ProductOrServiceAxis_01a010e8-d6ce-749a-9d60-486ebb2398d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-749a-9d60-486ebb2398d7_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a010e8-d6ce-749a-9d60-486ebb2398d7" xlink:to="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-749a-9d60-486ebb2398d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a010e8-d6ce-749a-9d60-486ebb2398d7" xlink:to="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ScheduledServiceRevenueMember_01a010e8-d6ce-78ed-b2cc-c31a0eb1048d" xlink:href="algt-20260930.xsd#algt_ScheduledServiceRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_algt_ScheduledServiceRevenueMember_01a010e8-d6ce-78ed-b2cc-c31a0eb1048d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirrelatedrevenueMember_01a010e8-d6ce-7f57-86ee-ac4636361019" xlink:href="algt-20260930.xsd#algt_AirrelatedrevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_algt_AirrelatedrevenueMember_01a010e8-d6ce-7f57-86ee-ac4636361019" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CobrandRevenueMember_01a010e8-d6ce-7202-b811-dc93a833aa47" xlink:href="algt-20260930.xsd#algt_CobrandRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_algt_CobrandRevenueMember_01a010e8-d6ce-7202-b811-dc93a833aa47" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RoomsMember_01a010e8-d6ce-7078-b514-2a8403363d08" xlink:href="algt-20260930.xsd#algt_RoomsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_algt_RoomsMember_01a010e8-d6ce-7078-b514-2a8403363d08" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FoodAndBeverageMember_01a010e8-d6ce-74f3-839e-1c47a129acbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FoodAndBeverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_us-gaap_FoodAndBeverageMember_01a010e8-d6ce-74f3-839e-1c47a129acbf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductAndServiceOtherMember_01a010e8-d6ce-7f0a-af69-ed8f5995123d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductAndServiceOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a010e8-d6ce-75d3-ad31-debd9d72661f" xlink:to="loc_us-gaap_ProductAndServiceOtherMember_01a010e8-d6ce-7f0a-af69-ed8f5995123d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7112-af92-5d1bb039a954" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc_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_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d6ce-71f6-8cb9-ee4b94b7df0f" 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_01a010e8-d6ce-7dd3-b09c-a8fe7e6be5cc" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d6ce-71f6-8cb9-ee4b94b7df0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PassengerRevenueMember_01a010e8-d6ce-74fc-864e-5d2455a92b87" xlink:href="algt-20260930.xsd#algt_PassengerRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d6ce-71f6-8cb9-ee4b94b7df0f" xlink:to="loc_algt_PassengerRevenueMember_01a010e8-d6ce-74fc-864e-5d2455a92b87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ResortRevenueMember_01a010e8-d6ce-77ba-8a80-bf627f4ace12" xlink:href="algt-20260930.xsd#algt_ResortRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d6ce-71f6-8cb9-ee4b94b7df0f" xlink:to="loc_algt_ResortRevenueMember_01a010e8-d6ce-77ba-8a80-bf627f4ace12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a010e8-d6ce-722d-8d78-a2651aa569f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a010e8-d6ce-7a29-abf7-55fb90200fcb" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a010e8-d6ce-722d-8d78-a2651aa569f9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/RevenueRecognitionNarrativeDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#RevenueRecognitionNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/RevenueRecognitionNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7f34-bd41-07253af2189b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7f34-bd41-07253af2189b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a1a000-0d9d-732c-985b-3e7d76cb7a09" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_01a010e8-d6ce-7f34-bd41-07253af2189b" xlink:to="loc_srt_ProductOrServiceAxis_01a1a000-0d9d-732c-985b-3e7d76cb7a09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a1a000-0d9d-732c-985b-3e7d76cb7a09_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a1a000-0d9d-732c-985b-3e7d76cb7a09" xlink:to="loc_srt_ProductsAndServicesDomain_01a1a000-0d9d-732c-985b-3e7d76cb7a09_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a1a000-0d9d-7dd9-8fd0-c99ea4da837d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a1a000-0d9d-732c-985b-3e7d76cb7a09" xlink:to="loc_srt_ProductsAndServicesDomain_01a1a000-0d9d-7dd9-8fd0-c99ea4da837d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ThirdPartyProductRevenueMember_01a1a000-0d9d-7a3d-825f-b06325b58737" xlink:href="algt-20260930.xsd#algt_ThirdPartyProductRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a1a000-0d9d-7dd9-8fd0-c99ea4da837d" xlink:to="loc_algt_ThirdPartyProductRevenueMember_01a1a000-0d9d-7a3d-825f-b06325b58737" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a19fff-dce1-72ac-9808-7cc6304e4662" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a19fff-dce1-72ac-9808-7cc6304e4662" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ContractwithCustomerLiabilityForwardBookings_01a19fff-dce1-7288-97fa-b462c9e8334c" xlink:href="algt-20260930.xsd#algt_ContractwithCustomerLiabilityForwardBookings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_algt_ContractwithCustomerLiabilityForwardBookings_01a19fff-dce1-7288-97fa-b462c9e8334c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ContractwithCustomerLiabilityCreditVoucherBookings_01a19fff-dce1-73a0-820b-5decc3328d49" xlink:href="algt-20260930.xsd#algt_ContractwithCustomerLiabilityCreditVoucherBookings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_algt_ContractwithCustomerLiabilityCreditVoucherBookings_01a19fff-dce1-73a0-820b-5decc3328d49" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_01a19fff-dce1-7097-aaf6-369959be6783" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuePerformanceObligationDescriptionOfTiming"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_01a19fff-dce1-7097-aaf6-369959be6783" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirTrafficLiabilityRecognized_01a19fff-dce1-72bd-83bc-219506418d09" xlink:href="algt-20260930.xsd#algt_AirTrafficLiabilityRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_algt_AirTrafficLiabilityRecognized_01a19fff-dce1-72bd-83bc-219506418d09" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueContractAssetAmount_01a19fff-f0f0-7d42-827e-fcf242cb3fdc" xlink:href="algt-20260930.xsd#algt_RevenueContractAssetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_algt_RevenueContractAssetAmount_01a19fff-f0f0-7d42-827e-fcf242cb3fdc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a1a000-0d9d-7fc2-8ce6-c00b8b3e4ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a19fff-dce1-75e5-80c3-eb696480589e" xlink:to="loc_us-gaap_Revenues_01a1a000-0d9d-7fc2-8ce6-c00b8b3e4ddc" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/PropertyandEquipmentDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#PropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/PropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" 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_01a010e8-d6ce-7980-9561-df402128ef1b" 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_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a010e8-d6ce-7980-9561-df402128ef1b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a010e8-d6ce-7607-bba1-838a0fc2e352" 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_01a010e8-d6ce-7980-9561-df402128ef1b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a010e8-d6ce-7607-bba1-838a0fc2e352" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a010e8-d6ce-7607-bba1-838a0fc2e352_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_01a010e8-d6ce-7607-bba1-838a0fc2e352" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a010e8-d6ce-7607-bba1-838a0fc2e352_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a010e8-d6ce-7b20-a1d8-64fe33e17c1f" 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_01a010e8-d6ce-7607-bba1-838a0fc2e352" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a010e8-d6ce-7b20-a1d8-64fe33e17c1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirlineMember_01a010e8-d6ce-7d7e-8590-be88906d28f5" xlink:href="algt-20260930.xsd#algt_AirlineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a010e8-d6ce-7b20-a1d8-64fe33e17c1f" xlink:to="loc_algt_AirlineMember_01a010e8-d6ce-7d7e-8590-be88906d28f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FlightEquipmentGross_01a010e8-d6ce-7810-8897-9f6dccb275dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FlightEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_FlightEquipmentGross_01a010e8-d6ce-7810-8897-9f6dccb275dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ComputerHardwareandSoftware_01a010e8-d6ce-741a-8607-b909c847700a" xlink:href="algt-20260930.xsd#algt_ComputerHardwareandSoftware"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_algt_ComputerHardwareandSoftware_01a010e8-d6ce-741a-8607-b909c847700a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_Landandbuildingsleaseholdimprovements_01a010e8-d6ce-7a06-b7c2-7a1d772b9f95" xlink:href="algt-20260930.xsd#algt_Landandbuildingsleaseholdimprovements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_algt_Landandbuildingsleaseholdimprovements_01a010e8-d6ce-7a06-b7c2-7a1d772b9f95" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentOther_01a010e8-d6ce-7dd5-a732-202d595fa236" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentOther_01a010e8-d6ce-7dd5-a732-202d595fa236" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_01a010e8-d6ce-7436-83c2-559bb92ab1d0" 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_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_01a010e8-d6ce-7436-83c2-559bb92ab1d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a010e8-d6ce-7068-b76c-8c1f40e51c76" 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_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a010e8-d6ce-7068-b76c-8c1f40e51c76" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_01a010e8-d6ce-7d2f-96c6-d16a3ab34f04" 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_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_01a010e8-d6ce-7d2f-96c6-d16a3ab34f04" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfAircraftCommittedToPurchase_01a010e8-d6ce-7850-8d9d-3023d64fc93f" xlink:href="algt-20260930.xsd#algt_NumberOfAircraftCommittedToPurchase"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a010e8-d6ce-7ed3-9a85-66e49f5ccf91" xlink:to="loc_algt_NumberOfAircraftCommittedToPurchase_01a010e8-d6ce-7850-8d9d-3023d64fc93f" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/LongTermDebtComponentsofLongTermDebtDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#LongTermDebtComponentsofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/LongTermDebtComponentsofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" 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_01a010e8-d6ce-7d2a-ac87-004fd721d03f" 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_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_DebtInstrumentTable_01a010e8-d6ce-7d2a-ac87-004fd721d03f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9" 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_01a010e8-d6ce-7d2a-ac87-004fd721d03f" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9_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_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" 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_01a010e8-d6ce-70dc-81ac-0ed6475bf2d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_FixedRateMember_01a010e8-d767-7bde-9220-d1611d85d828" xlink:href="algt-20260930.xsd#algt_FixedRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_FixedRateMember_01a010e8-d767-7bde-9220-d1611d85d828" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_VariableRateMember_01a010e8-d767-7257-86e7-65ebbd064bc2" xlink:href="algt-20260930.xsd#algt_VariableRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_VariableRateMember_01a010e8-d767-7257-86e7-65ebbd064bc2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredNotesDue2027Member_01a010e8-d767-7d21-b5d2-4bc303b2c4de" xlink:href="algt-20260930.xsd#algt_SeniorSecuredNotesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_SeniorSecuredNotesDue2027Member_01a010e8-d767-7d21-b5d2-4bc303b2c4de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ConsolidatedVariableInterestEntitiesDue2029Member_01a010e8-d767-7f32-b275-9542efb269ac" xlink:href="algt-20260930.xsd#algt_ConsolidatedVariableInterestEntitiesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_ConsolidatedVariableInterestEntitiesDue2029Member_01a010e8-d767-7f32-b275-9542efb269ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevolvingCreditFacilitiesDue2027Member_01a010e8-d767-7af1-9af7-5b0a0f134fd5" xlink:href="algt-20260930.xsd#algt_RevolvingCreditFacilitiesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_RevolvingCreditFacilitiesDue2027Member_01a010e8-d767-7af1-9af7-5b0a0f134fd5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember_01a010e8-d767-7e4e-bfab-9acd805c22c7" xlink:href="algt-20260930.xsd#algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember_01a010e8-d767-7e4e-bfab-9acd805c22c7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMember_01a010e8-d767-7fae-8895-4c636897a146" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_us-gaap_LongTermDebtMember_01a010e8-d767-7fae-8895-4c636897a146" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnhancedEquipmentTrustCertificateMember_01a010e8-d767-786b-bfc9-2139000014ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnhancedEquipmentTrustCertificateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a010e8-d767-777f-9352-2db22a2e7905" xlink:to="loc_us-gaap_EnhancedEquipmentTrustCertificateMember_01a010e8-d767-786b-bfc9-2139000014ab" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a010e8-d767-7e4b-bf8c-ba52d3fba443" 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_01a010e8-d6ce-7d2a-ac87-004fd721d03f" xlink:to="loc_srt_RangeAxis_01a010e8-d767-7e4b-bf8c-ba52d3fba443" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a010e8-d767-7e4b-bf8c-ba52d3fba443_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_01a010e8-d767-7e4b-bf8c-ba52d3fba443" xlink:to="loc_srt_RangeMember_01a010e8-d767-7e4b-bf8c-ba52d3fba443_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a010e8-d767-7efc-91ff-335552ca207b" 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_01a010e8-d767-7e4b-bf8c-ba52d3fba443" xlink:to="loc_srt_RangeMember_01a010e8-d767-7efc-91ff-335552ca207b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a010e8-d767-7174-92ae-346ed3ae3323" 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_01a010e8-d767-7efc-91ff-335552ca207b" xlink:to="loc_srt_MinimumMember_01a010e8-d767-7174-92ae-346ed3ae3323" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a010e8-d767-71fe-8657-4f2a412d77a4" 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_01a010e8-d767-7efc-91ff-335552ca207b" xlink:to="loc_srt_MaximumMember_01a010e8-d767-71fe-8657-4f2a412d77a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a010e8-d767-7e97-86ea-a605b105c108" 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_01a010e8-d6ce-7d2a-ac87-004fd721d03f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a010e8-d767-7e97-86ea-a605b105c108" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-7e97-86ea-a605b105c108_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_01a010e8-d767-7e97-86ea-a605b105c108" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-7e97-86ea-a605b105c108_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" 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_01a010e8-d767-7e97-86ea-a605b105c108" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_01a010e8-d767-7c49-8d02-cf1fc0ac8015" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" xlink:to="loc_us-gaap_SeniorNotesMember_01a010e8-d767-7c49-8d02-cf1fc0ac8015" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a010e8-d767-7691-a4be-8b42ace186f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" xlink:to="loc_us-gaap_SecuredDebtMember_01a010e8-d767-7691-a4be-8b42ace186f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a010e8-d767-7eca-ae9f-d06578e788d5" 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_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a010e8-d767-7eca-ae9f-d06578e788d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_01a010e8-d767-7bec-9a5b-2d504e6e89fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d767-74ae-b748-36657de72ebb" xlink:to="loc_us-gaap_UnsecuredDebtMember_01a010e8-d767-7bec-9a5b-2d504e6e89fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayable_01a010e8-d767-78e4-941d-bff4c72c6eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_NotesPayable_01a010e8-d767-78e4-941d-bff4c72c6eb3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredLongTermDebt_01a010e8-d767-730c-8ef3-ef1210c72cb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_SecuredLongTermDebt_01a010e8-d767-730c-8ef3-ef1210c72cb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_01a010e8-d767-71e0-9030-59884c16db1b" 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_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_LongTermDebtCurrent_01a010e8-d767-71e0-9030-59884c16db1b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_01a010e8-d767-78be-8366-818329ed16cd" 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_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_01a010e8-d767-78be-8366-818329ed16cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_01a010e8-d767-7b20-8eec-170b93b59c31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_01a010e8-d767-7b20-8eec-170b93b59c31" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a010e8-d767-7bb0-aad3-188bc02304ec" 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_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a010e8-d767-7bb0-aad3-188bc02304ec" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_01a010e8-d767-7ffa-9221-b475a21b53c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_FinanceLeaseLiability_01a010e8-d767-7ffa-9221-b475a21b53c3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_01a010e8-d767-717b-9750-9c9fb49abf43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_01a010e8-d767-717b-9750-9c9fb49abf43" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_01a010e8-d767-7e5a-bbce-63e99c4d909e" 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_01a010e8-d767-7d2e-a6a6-fba91dce568e" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_01a010e8-d767-7e5a-bbce-63e99c4d909e" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#LongTermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/LongTermDebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" 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_01a010e8-d767-70de-93c8-3da34af5aa5d" 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_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_DebtInstrumentTable_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a1a00a-fb9c-7597-9e30-2aea20b79d46" 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_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a1a00a-fb9c-7597-9e30-2aea20b79d46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a1a00a-fb9c-7597-9e30-2aea20b79d46_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_01a1a00a-fb9c-7597-9e30-2aea20b79d46" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a1a00a-fb9c-7597-9e30-2aea20b79d46_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a1a00a-fb9c-7b1b-a611-79055ed4cf28" 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_01a1a00a-fb9c-7597-9e30-2aea20b79d46" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a1a00a-fb9c-7b1b-a611-79055ed4cf28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a1a00a-fb9c-794d-9b48-df0c17f5bb53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a1a00a-fb9c-7b1b-a611-79055ed4cf28" xlink:to="loc_us-gaap_SecuredDebtMember_01a1a00a-fb9c-794d-9b48-df0c17f5bb53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a1a00a-fb9c-72d4-95b6-e5e7bb2d5672" 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_01a1a00a-fb9c-7b1b-a611-79055ed4cf28" xlink:to="loc_us-gaap_LineOfCreditMember_01a1a00a-fb9c-72d4-95b6-e5e7bb2d5672" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a1a00a-fb9c-7435-bac9-da9c1ef5d370" 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_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:to="loc_us-gaap_CreditFacilityAxis_01a1a00a-fb9c-7435-bac9-da9c1ef5d370" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a1a00a-fb9c-7435-bac9-da9c1ef5d370_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_01a1a00a-fb9c-7435-bac9-da9c1ef5d370" xlink:to="loc_us-gaap_CreditFacilityDomain_01a1a00a-fb9c-7435-bac9-da9c1ef5d370_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a1a00a-fb9c-79af-b8ee-846254ae1e80" 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_01a1a00a-fb9c-7435-bac9-da9c1ef5d370" xlink:to="loc_us-gaap_CreditFacilityDomain_01a1a00a-fb9c-79af-b8ee-846254ae1e80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a1a00a-fb9c-7036-91c9-41399037fdfc" 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_01a1a00a-fb9c-79af-b8ee-846254ae1e80" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a1a00a-fb9c-7036-91c9-41399037fdfc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredRevolvingLoanFacilityMember_01a1a00a-fb9c-75bb-8ab9-dc76d68f0eed" xlink:href="algt-20260930.xsd#algt_SeniorSecuredRevolvingLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a1a00a-fb9c-79af-b8ee-846254ae1e80" xlink:to="loc_algt_SeniorSecuredRevolvingLoanFacilityMember_01a1a00a-fb9c-75bb-8ab9-dc76d68f0eed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a1a00a-fb9c-7d82-b066-16dc474782c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_01a1a00a-fb9c-73d2-a25c-9f4290a9a4e9" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a1a00a-fb9c-7d82-b066-16dc474782c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BarclaysBankPLCMember_01a1a00a-fb9c-7c6a-b0d0-29f5aef5df83" xlink:href="algt-20260930.xsd#algt_BarclaysBankPLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a1a00a-fb9c-7d82-b066-16dc474782c6" xlink:to="loc_algt_BarclaysBankPLCMember_01a1a00a-fb9c-7c6a-b0d0-29f5aef5df83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a1a00a-fb9c-763d-8e3d-1211dcab6d54" 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_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a1a00a-fb9c-763d-8e3d-1211dcab6d54" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-763d-8e3d-1211dcab6d54_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_01a1a00a-fb9c-763d-8e3d-1211dcab6d54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-763d-8e3d-1211dcab6d54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-789c-b2a4-6531f5251d74" 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_01a1a00a-fb9c-763d-8e3d-1211dcab6d54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-789c-b2a4-6531f5251d74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementCANSBCANSB1Member_01a1a00a-fb9c-7bd2-a38a-d6fcf90783c7" xlink:href="algt-20260930.xsd#algt_CreditAgreementCANSBCANSB1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-789c-b2a4-6531f5251d74" xlink:to="loc_algt_CreditAgreementCANSBCANSB1Member_01a1a00a-fb9c-7bd2-a38a-d6fcf90783c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementDBJ2DBJ21Member_01a1a00a-fb9c-720d-820b-fe4922a52523" xlink:href="algt-20260930.xsd#algt_CreditAgreementDBJ2DBJ21Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-789c-b2a4-6531f5251d74" xlink:to="loc_algt_CreditAgreementDBJ2DBJ21Member_01a1a00a-fb9c-720d-820b-fe4922a52523" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OtherSecuredDebtMember_01a1a00b-1d33-71d4-9260-8a8b4e2ad1af" xlink:href="algt-20260930.xsd#algt_OtherSecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a1a00a-fb9c-789c-b2a4-6531f5251d74" xlink:to="loc_algt_OtherSecuredDebtMember_01a1a00b-1d33-71d4-9260-8a8b4e2ad1af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6" 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_01a010e8-d767-70de-93c8-3da34af5aa5d" xlink:to="loc_srt_RangeAxis_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6_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_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6" xlink:to="loc_srt_RangeMember_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a1a00a-fb9c-7ba9-b7c1-21758e7c5dd7" 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_01a1a00a-fb9c-7781-a8fd-5fa2e68ecbd6" xlink:to="loc_srt_RangeMember_01a1a00a-fb9c-7ba9-b7c1-21758e7c5dd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a1a00a-fb9c-7d22-8014-e046dab56b4a" 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_01a1a00a-fb9c-7ba9-b7c1-21758e7c5dd7" xlink:to="loc_srt_MinimumMember_01a1a00a-fb9c-7d22-8014-e046dab56b4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a1a00a-fb9c-7485-96ec-ae5b3d25eae4" 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_01a1a00a-fb9c-7ba9-b7c1-21758e7c5dd7" xlink:to="loc_srt_MaximumMember_01a1a00a-fb9c-7485-96ec-ae5b3d25eae4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_DebtInstrumentRedeemedAmount_01a1a00a-fb9c-70c8-bcb4-f77b944dbab1" xlink:href="algt-20260930.xsd#algt_DebtInstrumentRedeemedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_algt_DebtInstrumentRedeemedAmount_01a1a00a-fb9c-70c8-bcb4-f77b944dbab1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a1a00a-fb9c-7d0f-8deb-91e0cedffd96" 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_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a1a00a-fb9c-7d0f-8deb-91e0cedffd96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_01a1a00a-fb9c-7d47-8a1d-4e530b268eef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_LineOfCredit_01a1a00a-fb9c-7d47-8a1d-4e530b268eef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_01a1a00a-fb9c-7bf2-b8a5-7e0b48daea8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_01a1a00a-fb9c-7bf2-b8a5-7e0b48daea8f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a1a00a-fb9c-7ae0-8cbd-b8dc1eb0bb32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a1a00a-fb9c-7ae0-8cbd-b8dc1eb0bb32" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_01a1a00a-fb9c-77be-bf87-671910f55180" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_DebtInstrumentTerm_01a1a00a-fb9c-77be-bf87-671910f55180" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_01a1a00b-1d33-7ba2-a169-31a3a516179e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_01a1a00b-1d33-7ba2-a169-31a3a516179e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_01a1a00b-1d33-77ef-b691-165072d9ec22" 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_01a1a00a-fb9c-7c3e-80eb-e2385e12dc2b" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_01a1a00b-1d33-77ef-b691-165072d9ec22" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_01a010e8-d768-7f24-8ca6-d2947a5233a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_01a010e8-d768-7f24-8ca6-d2947a5233a3" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_01a010e8-d768-749d-acc6-3c8cf869d0b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_01a010e8-d768-749d-acc6-3c8cf869d0b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-749d-acc6-3c8cf869d0b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_01a010e8-d768-749d-acc6-3c8cf869d0b7" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-749d-acc6-3c8cf869d0b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_01a010e8-d768-749d-acc6-3c8cf869d0b7" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_01a010e8-d768-7b19-ae40-812510ffa22c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_MoneyMarketFundsMember_01a010e8-d768-7b19-ae40-812510ffa22c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_01a010e8-d768-7cd2-94cb-bb1eb470477c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_CommercialPaperMember_01a010e8-d768-7cd2-94cb-bb1eb470477c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_01a010e8-d768-77e5-b9e4-fed1fec0d201" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_01a010e8-d768-77e5-b9e4-fed1fec0d201" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryBondSecuritiesMember_01a010e8-d768-7716-8a5d-66cb158a456f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_USTreasuryBondSecuritiesMember_01a010e8-d768-7716-8a5d-66cb158a456f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a010e8-d768-790c-b25b-857e4df893d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a010e8-d768-790c-b25b-857e4df893d3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_01a010e8-d768-7d70-9a3d-2b2a6c3fa56a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_01a010e8-d768-7d70-9a3d-2b2a6c3fa56a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_01a010e8-d768-7675-993b-5ab96a5f9788" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_01a010e8-d768-784a-99ac-99ed03de1650" xlink:to="loc_us-gaap_CertificatesOfDepositMember_01a010e8-d768-7675-993b-5ab96a5f9788" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7ca4-aa8d-7cec508bac47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7ca4-aa8d-7cec508bac47" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7ca4-aa8d-7cec508bac47_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7ca4-aa8d-7cec508bac47" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7ca4-aa8d-7cec508bac47_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7773-a1f0-4c5f72834687" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7ca4-aa8d-7cec508bac47" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7773-a1f0-4c5f72834687" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_01a010e8-d768-791e-96d8-4f096201f0f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7773-a1f0-4c5f72834687" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_01a010e8-d768-791e-96d8-4f096201f0f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a010e8-d768-70f8-a41d-572cf0429df4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a010e8-d768-70f8-a41d-572cf0429df4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-70f8-a41d-572cf0429df4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a010e8-d768-70f8-a41d-572cf0429df4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-70f8-a41d-572cf0429df4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-7114-a798-639e693d249c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a010e8-d768-70f8-a41d-572cf0429df4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-7114-a798-639e693d249c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_01a010e8-d768-7409-80e7-cbeabad2e4be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-7114-a798-639e693d249c" xlink:to="loc_us-gaap_CashEquivalentsMember_01a010e8-d768-7409-80e7-cbeabad2e4be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_01a010e8-d768-7bbc-bafe-a6c0216eb32e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-7114-a798-639e693d249c" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_01a010e8-d768-7bbc-bafe-a6c0216eb32e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_LongTermInvestmentsMember_01a010e8-d768-7e59-a4b0-d7aedd270ee0" xlink:href="algt-20260930.xsd#algt_LongTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a010e8-d768-7114-a798-639e693d249c" xlink:to="loc_algt_LongTermInvestmentsMember_01a010e8-d768-7e59-a4b0-d7aedd270ee0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7220-afc1-b5e8c1817767" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_01a010e8-d768-7e90-a4dd-fe0d3071c78e" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7220-afc1-b5e8c1817767" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7220-afc1-b5e8c1817767_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7220-afc1-b5e8c1817767" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7220-afc1-b5e8c1817767_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-795c-84c1-c184a9615c5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7220-afc1-b5e8c1817767" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-795c-84c1-c184a9615c5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_01a010e8-d768-7da0-b4a9-1b4e3df5a050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-795c-84c1-c184a9615c5f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_01a010e8-d768-7da0-b4a9-1b4e3df5a050" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_01a010e8-d768-75ad-a09c-3570657cbcfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-795c-84c1-c184a9615c5f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_01a010e8-d768-75ad-a09c-3570657cbcfd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a010e8-d768-730c-98ca-8415eb94d266" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_01a010e8-d768-7f24-8ca6-d2947a5233a3" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a010e8-d768-730c-98ca-8415eb94d266" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a010e8-d768-7933-90f6-d4b996c5cbba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_01a010e8-d768-7f24-8ca6-d2947a5233a3" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a010e8-d768-7933-90f6-d4b996c5cbba" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/FairValueMeasurementsEstimatedFairValueofDebtDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#FairValueMeasurementsEstimatedFairValueofDebtDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/FairValueMeasurementsEstimatedFairValueofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_01a010e8-d768-7046-a717-f87d9fa4d241" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_01a010e8-d768-745a-ba99-4ab53de16a54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_01a010e8-d768-7046-a717-f87d9fa4d241" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_01a010e8-d768-745a-ba99-4ab53de16a54" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a010e8-d768-794d-8544-f37eb10ca4e6" 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_FairValueByBalanceSheetGroupingTable_01a010e8-d768-745a-ba99-4ab53de16a54" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a010e8-d768-794d-8544-f37eb10ca4e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d768-794d-8544-f37eb10ca4e6_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_01a010e8-d768-794d-8544-f37eb10ca4e6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d768-794d-8544-f37eb10ca4e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d768-7c12-8af0-427f6c7b419a" 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_01a010e8-d768-794d-8544-f37eb10ca4e6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d768-7c12-8af0-427f6c7b419a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NonPubliclyHeldDebtMember_01a010e8-d768-78fc-a0cc-eb75e1878d39" xlink:href="algt-20260930.xsd#algt_NonPubliclyHeldDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a010e8-d768-7c12-8af0-427f6c7b419a" xlink:to="loc_algt_NonPubliclyHeldDebtMember_01a010e8-d768-78fc-a0cc-eb75e1878d39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7bc0-8833-fbd709370d44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_01a010e8-d768-745a-ba99-4ab53de16a54" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7bc0-8833-fbd709370d44" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7bc0-8833-fbd709370d44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7bc0-8833-fbd709370d44" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7bc0-8833-fbd709370d44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7125-8203-f2ba096e12bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a010e8-d768-7bc0-8833-fbd709370d44" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7125-8203-f2ba096e12bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a010e8-d768-798d-8c43-812368e63c4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a010e8-d768-7125-8203-f2ba096e12bd" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a010e8-d768-798d-8c43-812368e63c4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7e98-beb2-76ddb8a9d369" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_01a010e8-d768-745a-ba99-4ab53de16a54" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7e98-beb2-76ddb8a9d369" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7e98-beb2-76ddb8a9d369_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7e98-beb2-76ddb8a9d369" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-7e98-beb2-76ddb8a9d369_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-70f9-bf35-1bae1f4135e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a010e8-d768-7e98-beb2-76ddb8a9d369" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-70f9-bf35-1bae1f4135e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_01a010e8-d768-7964-b4fd-ff284c1d676a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a010e8-d768-70f9-bf35-1bae1f4135e2" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_01a010e8-d768-7964-b4fd-ff284c1d676a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredLongTermDebt_01a010e8-d768-7897-bad1-765cff7d1ee4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_01a010e8-d768-7046-a717-f87d9fa4d241" xlink:to="loc_us-gaap_SecuredLongTermDebt_01a010e8-d768-7897-bad1-765cff7d1ee4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_01a010e8-d768-710c-90f4-9904f8aa2fee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_01a010e8-d768-7046-a717-f87d9fa4d241" xlink:to="loc_us-gaap_LongTermDebtFairValue_01a010e8-d768-710c-90f4-9904f8aa2fee" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SegmentsScheduleofSegmentReportingInformationbySegmentDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#SegmentsScheduleofSegmentReportingInformationbySegmentDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/SegmentsScheduleofSegmentReportingInformationbySegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" 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_01a010e8-d768-7a20-bd6b-10de7dcd10cb" 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_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a010e8-d768-7a20-bd6b-10de7dcd10cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d768-794c-a25b-636fb82e6d31" 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_01a010e8-d768-7a20-bd6b-10de7dcd10cb" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a010e8-d768-794c-a25b-636fb82e6d31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d768-794c-a25b-636fb82e6d31_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_01a010e8-d768-794c-a25b-636fb82e6d31" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d768-794c-a25b-636fb82e6d31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a010e8-d768-7437-913c-89a4788faef6" 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_01a010e8-d768-794c-a25b-636fb82e6d31" xlink:to="loc_us-gaap_SegmentDomain_01a010e8-d768-7437-913c-89a4788faef6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AllegiantAirMember_01a010e8-d768-7478-a4b1-e7cb25dabf87" xlink:href="algt-20260930.xsd#algt_AllegiantAirMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d768-7437-913c-89a4788faef6" xlink:to="loc_algt_AllegiantAirMember_01a010e8-d768-7478-a4b1-e7cb25dabf87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryAirlinesMember_01a010e8-d768-7cd0-a086-aede41daa8e7" xlink:href="algt-20260930.xsd#algt_SunCountryAirlinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d768-7437-913c-89a4788faef6" xlink:to="loc_algt_SunCountryAirlinesMember_01a010e8-d768-7cd0-a086-aede41daa8e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunseekerResortsMember_01a010e8-d768-7d4e-a81e-cb238e71b82b" xlink:href="algt-20260930.xsd#algt_SunseekerResortsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a010e8-d768-7437-913c-89a4788faef6" xlink:to="loc_algt_SunseekerResortsMember_01a010e8-d768-7d4e-a81e-cb238e71b82b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeAxis_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0" xlink:href="algt-20260930.xsd#algt_RevenueTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a010e8-d768-7a20-bd6b-10de7dcd10cb" xlink:to="loc_algt_RevenueTypeAxis_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeDomain_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0_default" xlink:href="algt-20260930.xsd#algt_RevenueTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_algt_RevenueTypeAxis_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0" xlink:to="loc_algt_RevenueTypeDomain_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeDomain_01a010e8-d768-7e82-8c3c-f80f2ee947a0" xlink:href="algt-20260930.xsd#algt_RevenueTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_algt_RevenueTypeAxis_01a010e8-d768-7aaa-80f6-68b0d8ddfdf0" xlink:to="loc_algt_RevenueTypeDomain_01a010e8-d768-7e82-8c3c-f80f2ee947a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ScheduledServiceMember_01a010e8-d768-78d0-862a-9483cd160c65" xlink:href="algt-20260930.xsd#algt_ScheduledServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_01a010e8-d768-7e82-8c3c-f80f2ee947a0" xlink:to="loc_algt_ScheduledServiceMember_01a010e8-d768-78d0-862a-9483cd160c65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AncillaryAirRelatedChargesMember_01a010e8-d768-7644-8532-4996735b22bd" xlink:href="algt-20260930.xsd#algt_AncillaryAirRelatedChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_01a010e8-d768-7e82-8c3c-f80f2ee947a0" xlink:to="loc_algt_AncillaryAirRelatedChargesMember_01a010e8-d768-7644-8532-4996735b22bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_LoyaltyRedemptionsMember_01a010e8-d768-7963-9fea-d74873ee67a5" xlink:href="algt-20260930.xsd#algt_LoyaltyRedemptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_01a010e8-d768-7e82-8c3c-f80f2ee947a0" xlink:to="loc_algt_LoyaltyRedemptionsMember_01a010e8-d768-7963-9fea-d74873ee67a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_01a010e8-d768-78e0-9f65-1b012bd1ef89" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a010e8-d768-7a20-bd6b-10de7dcd10cb" xlink:to="loc_srt_ConsolidationItemsAxis_01a010e8-d768-78e0-9f65-1b012bd1ef89" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_01a010e8-d768-78e0-9f65-1b012bd1ef89_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_01a010e8-d768-78e0-9f65-1b012bd1ef89" xlink:to="loc_srt_ConsolidationItemsDomain_01a010e8-d768-78e0-9f65-1b012bd1ef89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_01a010e8-d768-7ac7-97c2-ae036e054c7a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_01a010e8-d768-78e0-9f65-1b012bd1ef89" xlink:to="loc_srt_ConsolidationItemsDomain_01a010e8-d768-7ac7-97c2-ae036e054c7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_01a010e8-d768-7b9e-9dbe-9e17c0422614" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_01a010e8-d768-7ac7-97c2-ae036e054c7a" xlink:to="loc_us-gaap_OperatingSegmentsMember_01a010e8-d768-7b9e-9dbe-9e17c0422614" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a010e8-d768-7e6b-9ca4-bcd51c9ecf6f" 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_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a010e8-d768-7e6b-9ca4-bcd51c9ecf6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ThirdPartyProducts_01a010e8-d768-7caa-802a-192312606fa3" xlink:href="algt-20260930.xsd#algt_ThirdPartyProducts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_ThirdPartyProducts_01a010e8-d768-7caa-802a-192312606fa3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_FixedFeeContractRevenue_01a010e8-d768-7fd9-bb96-2d9b7a30f439" xlink:href="algt-20260930.xsd#algt_FixedFeeContractRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_FixedFeeContractRevenue_01a010e8-d768-7fd9-bb96-2d9b7a30f439" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CargoRevenue_01a010e8-d768-7ab8-9a42-edbefd3deed4" xlink:href="algt-20260930.xsd#algt_CargoRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_CargoRevenue_01a010e8-d768-7ab8-9a42-edbefd3deed4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OtherOperatingRevenue_01a010e8-d768-7fdd-a1eb-7f6bd9222fd5" xlink:href="algt-20260930.xsd#algt_OtherOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_OtherOperatingRevenue_01a010e8-d768-7fdd-a1eb-7f6bd9222fd5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a010e8-d768-72fb-a77a-4ef6983305f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_Revenues_01a010e8-d768-72fb-a77a-4ef6983305f6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_01a010e8-d768-7dc2-a058-c95b7fd6e9b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LaborAndRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_LaborAndRelatedExpense_01a010e8-d768-7dc2-a058-c95b7fd6e9b0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AircraftFuel_01a010e8-d768-7720-9275-b132a4f1fc25" xlink:href="algt-20260930.xsd#algt_AircraftFuel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_AircraftFuel_01a010e8-d768-7720-9275-b132a4f1fc25" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_StationOperations_01a010e8-d768-7162-988b-bef9530563a6" xlink:href="algt-20260930.xsd#algt_StationOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_StationOperations_01a010e8-d768-7162-988b-bef9530563a6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_01a010e8-d768-7260-aa22-b008e3cb3df0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_DepreciationAndAmortization_01a010e8-d768-7260-aa22-b008e3cb3df0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AircraftMaintenanceMaterialsAndRepairs_01a010e8-d768-7086-8176-11392bc8b2a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AircraftMaintenanceMaterialsAndRepairs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_AircraftMaintenanceMaterialsAndRepairs_01a010e8-d768-7086-8176-11392bc8b2a7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_01a010e8-d768-7d44-86bf-c2dfa2c43a0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_SellingAndMarketingExpense_01a010e8-d768-7d44-86bf-c2dfa2c43a0b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AircraftLeaseRental_01a010e8-d768-7864-8baa-e90815393e5c" xlink:href="algt-20260930.xsd#algt_AircraftLeaseRental"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_algt_AircraftLeaseRental_01a010e8-d768-7864-8baa-e90815393e5c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostAndExpenseOperating_01a010e8-d768-7811-9e98-dea238917a29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCostAndExpenseOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_OtherCostAndExpenseOperating_01a010e8-d768-7811-9e98-dea238917a29" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_01a010e8-d768-7ebf-a4d2-8ebffabd9228" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringSettlementAndImpairmentProvisions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_01a010e8-d768-7ebf-a4d2-8ebffabd9228" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_01a010e8-d768-7f26-b5ac-ebc6dff249c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_OperatingExpenses_01a010e8-d768-7f26-b5ac-ebc6dff249c3" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_01a010e8-d768-70e9-b8b1-62c1657eadb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_OperatingIncomeLoss_01a010e8-d768-70e9-b8b1-62c1657eadb6" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNet_01a010e8-d768-7665-9017-cf342160fafb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_InterestIncomeExpenseNet_01a010e8-d768-7665-9017-cf342160fafb" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_01a010e8-d768-78e9-a956-d7b0d3d8b8cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_InterestExpense_01a010e8-d768-78e9-a956-d7b0d3d8b8cf" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_01a010e8-d768-7438-99af-59572539db91" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestCostsCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_InterestCostsCapitalized_01a010e8-d768-7438-99af-59572539db91" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_01a010e8-d768-732e-b18f-82a38eb47ba2" 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_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_01a010e8-d768-732e-b18f-82a38eb47ba2" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a010e8-d768-7cc2-95b2-c282fce1a650" 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_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a010e8-d768-7cc2-95b2-c282fce1a650" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_01a010e8-d768-7956-a972-eef63589f76c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_01a010e8-d768-7956-a972-eef63589f76c" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_01a010e8-d768-74fb-b10b-5c5375221762" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a010e8-d768-7e9e-8249-36532fcdb44c" xlink:to="loc_us-gaap_Assets_01a010e8-d768-74fb-b10b-5c5375221762" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryNarrativeDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#AcquisitionofSunCountryNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" 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_01a010e8-d768-7e98-a598-70997168b4b6" 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_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7e98-a598-70997168b4b6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a19ffd-36a3-7c84-b6e8-b4f47f80b427" 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_01a010e8-d768-7e98-a598-70997168b4b6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a19ffd-36a3-7c84-b6e8-b4f47f80b427" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a19ffd-36a3-7c84-b6e8-b4f47f80b427_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_01a19ffd-36a3-7c84-b6e8-b4f47f80b427" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a19ffd-36a3-7c84-b6e8-b4f47f80b427_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a19ffd-36a3-78c4-867e-c86c7ecfa5ad" 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_01a19ffd-36a3-7c84-b6e8-b4f47f80b427" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a19ffd-36a3-78c4-867e-c86c7ecfa5ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_01a19ffd-36a3-7eca-b714-b29fa15d7668" xlink:href="algt-20260930.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a19ffd-36a3-78c4-867e-c86c7ecfa5ad" xlink:to="loc_algt_SunCountryMember_01a19ffd-36a3-7eca-b714-b29fa15d7668" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a19ffd-6287-7fbd-83cc-8f52ea4a614a" 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_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7e98-a598-70997168b4b6" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a19ffd-6287-7fbd-83cc-8f52ea4a614a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a19ffd-6287-7fbd-83cc-8f52ea4a614a_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_01a19ffd-6287-7fbd-83cc-8f52ea4a614a" xlink:to="loc_us-gaap_ClassOfStockDomain_01a19ffd-6287-7fbd-83cc-8f52ea4a614a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a19ffd-6287-738c-9360-32c3f3f08d93" 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_01a19ffd-6287-7fbd-83cc-8f52ea4a614a" xlink:to="loc_us-gaap_ClassOfStockDomain_01a19ffd-6287-738c-9360-32c3f3f08d93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_01a19ffd-6287-7239-abf9-74dfb56eec99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a19ffd-6287-738c-9360-32c3f3f08d93" xlink:to="loc_us-gaap_RestrictedStockMember_01a19ffd-6287-7239-abf9-74dfb56eec99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationByTypeAxis_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7" 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_01a010e8-d768-7e98-a598-70997168b4b6" xlink:to="loc_us-gaap_ContingentConsiderationByTypeAxis_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7_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_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-77a9-917f-adb4adc7401e" 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_01a19ffd-6287-7aa0-8c7c-4a32ec50f1e7" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-77a9-917f-adb4adc7401e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PurchaseConsiderationMember_01a19ffd-6287-7c6d-a91e-995f1f47a25f" xlink:href="algt-20260930.xsd#algt_PurchaseConsiderationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-77a9-917f-adb4adc7401e" xlink:to="loc_algt_PurchaseConsiderationMember_01a19ffd-6287-7c6d-a91e-995f1f47a25f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SpecialChargesMember_01a19ffd-6287-72b5-b416-c61db8dc3e09" xlink:href="algt-20260930.xsd#algt_SpecialChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_01a19ffd-6287-77a9-917f-adb4adc7401e" xlink:to="loc_algt_SpecialChargesMember_01a19ffd-6287-72b5-b416-c61db8dc3e09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_01a19ffd-36a3-7d25-9cb4-1e34d82eeb40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_01a19ffd-36a3-7d25-9cb4-1e34d82eeb40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionSharePrice_01a19ffd-36a3-74f0-9f16-c3456d31cad0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionSharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_BusinessAcquisitionSharePrice_01a19ffd-36a3-74f0-9f16-c3456d31cad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare_01a19ffd-36a3-7569-b5f1-f43aa2daa612" xlink:href="algt-20260930.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare_01a19ffd-36a3-7569-b5f1-f43aa2daa612" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare_01a19ffd-36a3-7545-b90b-b867801540fb" xlink:href="algt-20260930.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare_01a19ffd-36a3-7545-b90b-b867801540fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare_01a19ffd-36a3-7417-affd-52d6d03afb9a" xlink:href="algt-20260930.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare_01a19ffd-36a3-7417-affd-52d6d03afb9a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_01a19ffd-6287-7edd-bb56-07ce2ed91615" 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_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_01a19ffd-6287-7edd-bb56-07ce2ed91615" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_01a19ffd-6287-7814-8b42-e546c494afa1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_01a19ffd-6287-7814-8b42-e546c494afa1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfCommonStockSharesIssued_01a19ffd-6287-7a67-824b-f7338f350722" xlink:href="algt-20260930.xsd#algt_BusinessCombinationNumberOfCommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationNumberOfCommonStockSharesIssued_01a19ffd-6287-7a67-824b-f7338f350722" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_01a19ffd-6287-750d-a367-d4c45c1d6629" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_01a19ffd-6287-750d-a367-d4c45c1d6629" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod_01a19ffd-6287-7428-bafc-56fa9a50dcf6" xlink:href="algt-20260930.xsd#algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod_01a19ffd-6287-7428-bafc-56fa9a50dcf6" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConversionOfStockSharesIssued_01a19ffd-6287-7abc-8448-0da796f0df28" xlink:href="algt-20260930.xsd#algt_BusinessCombinationConversionOfStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_algt_BusinessCombinationConversionOfStockSharesIssued_01a19ffd-6287-7abc-8448-0da796f0df28" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_01a19ffd-6287-73ec-a077-e1c2ef25334a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a19ffd-36a3-7c83-9e9a-27eb3d08445a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_01a19ffd-6287-73ec-a077-e1c2ef25334a" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationContingentConsiderationDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#AcquisitionofSunCountryBusinessCombinationContingentConsiderationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationContingentConsiderationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7911-8486-805f8f3c22c4" 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_01a010e8-d768-7f4c-bf68-730710d20973" 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_01a010e8-d768-7911-8486-805f8f3c22c4" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7f4c-bf68-730710d20973" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7a56-8a7e-5b1293281126" 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_01a010e8-d768-7f4c-bf68-730710d20973" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7a56-8a7e-5b1293281126" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7a56-8a7e-5b1293281126_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_01a010e8-d768-7a56-8a7e-5b1293281126" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7a56-8a7e-5b1293281126_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7d53-b291-a6d50a2c4d84" 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_01a010e8-d768-7a56-8a7e-5b1293281126" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7d53-b291-a6d50a2c4d84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_01a010e8-d768-7a51-885c-ffe5ce465b8b" xlink:href="algt-20260930.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7d53-b291-a6d50a2c4d84" xlink:to="loc_algt_SunCountryMember_01a010e8-d768-7a51-885c-ffe5ce465b8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a010e8-d768-7fee-83a3-468284b50ff8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7f4c-bf68-730710d20973" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a010e8-d768-7fee-83a3-468284b50ff8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-7fee-83a3-468284b50ff8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a010e8-d768-7fee-83a3-468284b50ff8" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-7fee-83a3-468284b50ff8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a010e8-d768-7fee-83a3-468284b50ff8" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a010e8-d768-7ffc-adde-bafcdc47eb85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:to="loc_us-gaap_CommonStockMember_01a010e8-d768-7ffc-adde-bafcdc47eb85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember_01a010e8-d768-7bf7-ab43-a7446f9af74d" xlink:href="algt-20260930.xsd#algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:to="loc_algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember_01a010e8-d768-7bf7-ab43-a7446f9af74d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ConsiderationPaidWithRespectToAmazonMember_01a010e8-d768-7edd-8b6d-29413a346a20" xlink:href="algt-20260930.xsd#algt_ConsiderationPaidWithRespectToAmazonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:to="loc_algt_ConsiderationPaidWithRespectToAmazonMember_01a010e8-d768-7edd-8b6d-29413a346a20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_TerminationPaymentUnderTaxReceivableAgreementMember_01a010e8-d768-79da-9dd4-8497e9c69895" xlink:href="algt-20260930.xsd#algt_TerminationPaymentUnderTaxReceivableAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a010e8-d768-71d1-a5a0-d4f5d08f3eee" xlink:to="loc_algt_TerminationPaymentUnderTaxReceivableAgreementMember_01a010e8-d768-79da-9dd4-8497e9c69895" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a010e8-d768-7abb-ac0f-40ad27d4cdbd" 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_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7f4c-bf68-730710d20973" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a010e8-d768-7abb-ac0f-40ad27d4cdbd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a010e8-d768-7abb-ac0f-40ad27d4cdbd_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_01a010e8-d768-7abb-ac0f-40ad27d4cdbd" xlink:to="loc_us-gaap_ClassOfStockDomain_01a010e8-d768-7abb-ac0f-40ad27d4cdbd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a010e8-d768-751f-b64c-1a26d86f9b8e" 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_01a010e8-d768-7abb-ac0f-40ad27d4cdbd" xlink:to="loc_us-gaap_ClassOfStockDomain_01a010e8-d768-751f-b64c-1a26d86f9b8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_01a010e8-d768-7a7e-b005-fc5e19a31a70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a010e8-d768-751f-b64c-1a26d86f9b8e" xlink:to="loc_us-gaap_RestrictedStockMember_01a010e8-d768-7a7e-b005-fc5e19a31a70" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_01a010e8-d768-79a1-98bf-59f1e5e1b078" 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_01a010e8-d768-7911-8486-805f8f3c22c4" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_01a010e8-d768-79a1-98bf-59f1e5e1b078" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_01a010e8-d768-7e63-9adc-cd48a4b812dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7911-8486-805f8f3c22c4" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_01a010e8-d768-7e63-9adc-cd48a4b812dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_01a010e8-d768-7e0b-9643-cbb90f1ec94e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7911-8486-805f8f3c22c4" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_01a010e8-d768-7e0b-9643-cbb90f1ec94e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationPreliminaryAllocationDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#AcquisitionofSunCountryBusinessCombinationPreliminaryAllocationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationPreliminaryAllocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" 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_01a010e8-d768-741b-b869-1c23b16c4374" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-741b-b869-1c23b16c4374" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7602-95a8-826bce14d385" 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_01a010e8-d768-741b-b869-1c23b16c4374" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7602-95a8-826bce14d385" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7602-95a8-826bce14d385_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_01a010e8-d768-7602-95a8-826bce14d385" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7602-95a8-826bce14d385_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7c8b-b720-16dcd24f1524" 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_01a010e8-d768-7602-95a8-826bce14d385" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7c8b-b720-16dcd24f1524" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_01a010e8-d768-7377-b38a-949e10bfd946" xlink:href="algt-20260930.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7c8b-b720-16dcd24f1524" xlink:to="loc_algt_SunCountryMember_01a010e8-d768-7377-b38a-949e10bfd946" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a010e8-d768-7131-996d-93b9fcf99a9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-741b-b869-1c23b16c4374" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a010e8-d768-7131-996d-93b9fcf99a9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7131-996d-93b9fcf99a9c_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_01a010e8-d768-7131-996d-93b9fcf99a9c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7131-996d-93b9fcf99a9c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-71c4-9568-8dbbdf4c256f" 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_01a010e8-d768-7131-996d-93b9fcf99a9c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-71c4-9568-8dbbdf4c256f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_01a010e8-d768-705a-8c5f-6f0c8fc46494" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-71c4-9568-8dbbdf4c256f" xlink:to="loc_us-gaap_TrademarksMember_01a010e8-d768-705a-8c5f-6f0c8fc46494" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_01a010e8-d768-76c5-a049-dbe571230f83" 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_01a010e8-d768-71c4-9568-8dbbdf4c256f" xlink:to="loc_us-gaap_CustomerRelationshipsMember_01a010e8-d768-76c5-a049-dbe571230f83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_01a010e8-d768-75c8-ae70-f545581bef6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_01a010e8-d768-75c8-ae70-f545581bef6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash_01a010e8-d768-7b89-9ca7-05c44a1ca590" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash_01a010e8-d768-7b89-9ca7-05c44a1ca590" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment_01a010e8-d768-7d5e-8d70-ac5d40a2a14a" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment_01a010e8-d768-7d5e-8d70-ac5d40a2a14a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_01a010e8-d768-7172-b112-b0506cfacb1d" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_01a010e8-d768-7172-b112-b0506cfacb1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits_01a010e8-d768-7ce1-b26c-35148b63c97b" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits_01a010e8-d768-7ce1-b26c-35148b63c97b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet_01a010e8-d768-78e6-b0d4-26293c7031fa" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet_01a010e8-d768-78e6-b0d4-26293c7031fa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets_01a010e8-d768-7cfe-95cf-39b370429253" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets_01a010e8-d768-7cfe-95cf-39b370429253" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_01a010e8-d768-7403-8f2d-4a0cbcda088a" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_01a010e8-d768-7403-8f2d-4a0cbcda088a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_01a010e8-d768-7d32-b18b-316bdec1175e" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_Goodwill_01a010e8-d768-7d32-b18b-316bdec1175e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_01a010e8-d768-7423-a7c2-b23839ee0aab" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_01a010e8-d768-7423-a7c2-b23839ee0aab" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment_01a010e8-d768-713e-a9c7-57cc9dc7e30a" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment_01a010e8-d768-713e-a9c7-57cc9dc7e30a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet_01a010e8-d768-7f52-9cb0-75bdbf8a5d07" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet_01a010e8-d768-7f52-9cb0-75bdbf8a5d07" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets_01a010e8-d768-74fd-bc32-f203777a743a" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets_01a010e8-d768-74fd-bc32-f203777a743a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_01a010e8-d768-7750-8e86-89481999340e" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_01a010e8-d768-7750-8e86-89481999340e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_01a010e8-d768-7044-8e3e-95e0e08faedb" 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_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_01a010e8-d768-7044-8e3e-95e0e08faedb" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_01a010e8-d768-7335-8496-b771bcde6be5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_01a010e8-d768-7335-8496-b771bcde6be5" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent_01a010e8-d768-7c68-8a08-1b28367aa0d5" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent_01a010e8-d768-7c68-8a08-1b28367aa0d5" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability_01a010e8-d768-783b-9002-04cb3b78fd73" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability_01a010e8-d768-783b-9002-04cb3b78fd73" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability_01a010e8-d768-7d87-940a-de78a2d3aec4" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability_01a010e8-d768-7d87-940a-de78a2d3aec4" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts_01a010e8-d768-70ca-aa5e-d33b306ad643" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts_01a010e8-d768-70ca-aa5e-d33b306ad643" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts_01a010e8-d768-78e6-98eb-fb5097538573" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts_01a010e8-d768-78e6-98eb-fb5097538573" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_01a010e8-d768-785e-ad87-901020c0c689" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_01a010e8-d768-785e-ad87-901020c0c689" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent_01a010e8-d768-72c0-9f32-bb741df0d36a" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent_01a010e8-d768-72c0-9f32-bb741df0d36a" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent_01a010e8-d768-74c9-9aeb-bd18f7d335dc" xlink:href="algt-20260930.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent_01a010e8-d768-74c9-9aeb-bd18f7d335dc" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_01a010e8-d768-7fd1-8fde-fb9d80833e3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_01a010e8-d768-7fd1-8fde-fb9d80833e3c" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_01a010e8-d768-7bfb-85d3-49b090baaf93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_01a010e8-d768-7bfb-85d3-49b090baaf93" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_01a010e8-d768-7fd5-b923-27e90a5b1dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-746b-b1a9-8b40ade4c3d4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_01a010e8-d768-7fd5-b923-27e90a5b1dbf" xlink:type="arc" order="26"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#AcquisitionofSunCountryBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7eb2-8f55-a77efce84e68" 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_01a010e8-d768-7df6-95fa-c4f43fb0c3be" 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_01a010e8-d768-7eb2-8f55-a77efce84e68" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7df6-95fa-c4f43fb0c3be" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7d63-bfea-03b0b134f5b4" 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_01a010e8-d768-7df6-95fa-c4f43fb0c3be" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-7d63-bfea-03b0b134f5b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7d63-bfea-03b0b134f5b4_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_01a010e8-d768-7d63-bfea-03b0b134f5b4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-7d63-bfea-03b0b134f5b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-73ec-bc95-eff6735b81e1" 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_01a010e8-d768-7d63-bfea-03b0b134f5b4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-73ec-bc95-eff6735b81e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_01a010e8-d768-766d-806c-508ef8ebcadd" xlink:href="algt-20260930.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-73ec-bc95-eff6735b81e1" xlink:to="loc_algt_SunCountryMember_01a010e8-d768-766d-806c-508ef8ebcadd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a010e8-d768-7fbd-b0a0-acc3396dc2b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-7df6-95fa-c4f43fb0c3be" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a010e8-d768-7fbd-b0a0-acc3396dc2b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7fbd-b0a0-acc3396dc2b3_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_01a010e8-d768-7fbd-b0a0-acc3396dc2b3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7fbd-b0a0-acc3396dc2b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7d52-a153-7e7201b3bcff" 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_01a010e8-d768-7fbd-b0a0-acc3396dc2b3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7d52-a153-7e7201b3bcff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_01a010e8-d768-7581-96a3-a0bdef080498" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a010e8-d768-7d52-a153-7e7201b3bcff" xlink:to="loc_us-gaap_TrademarksMember_01a010e8-d768-7581-96a3-a0bdef080498" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_01a010e8-d768-7aad-b2d8-ea1b211e769c" 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_01a010e8-d768-7d52-a153-7e7201b3bcff" xlink:to="loc_us-gaap_CustomerRelationshipsMember_01a010e8-d768-7aad-b2d8-ea1b211e769c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_01a010e8-d768-7287-845d-47c2b48413d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7eb2-8f55-a77efce84e68" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_01a010e8-d768-7287-845d-47c2b48413d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_01a010e8-d768-79ae-8428-d615880d7f73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7eb2-8f55-a77efce84e68" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_01a010e8-d768-79ae-8428-d615880d7f73" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationProFormaInformationDetails" xlink:type="simple" xlink:href="algt-20260930.xsd#AcquisitionofSunCountryBusinessCombinationProFormaInformationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryBusinessCombinationProFormaInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7a4f-b2d3-ff41bb088cb8" 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_01a010e8-d768-77cf-bc0a-769cff3f0f4f" 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_01a010e8-d768-7a4f-b2d3-ff41bb088cb8" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a010e8-d768-77cf-bc0a-769cff3f0f4f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-74ed-b417-208a1c3c6d2d" 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_01a010e8-d768-77cf-bc0a-769cff3f0f4f" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a010e8-d768-74ed-b417-208a1c3c6d2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-74ed-b417-208a1c3c6d2d_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_01a010e8-d768-74ed-b417-208a1c3c6d2d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-74ed-b417-208a1c3c6d2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-749e-9646-6b8ac1e99f3f" 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_01a010e8-d768-74ed-b417-208a1c3c6d2d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-749e-9646-6b8ac1e99f3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_01a010e8-d768-7390-9493-cb4249ce5c67" xlink:href="algt-20260930.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a010e8-d768-749e-9646-6b8ac1e99f3f" xlink:to="loc_algt_SunCountryMember_01a010e8-d768-7390-9493-cb4249ce5c67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_01a010e8-d768-74aa-b1e3-53dd9a1811a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7a4f-b2d3-ff41bb088cb8" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_01a010e8-d768-74aa-b1e3-53dd9a1811a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_01a010e8-d768-7d9e-864e-e0149e7278e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a010e8-d768-7a4f-b2d3-ff41bb088cb8" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_01a010e8-d768-7d9e-864e-e0149e7278e3" xlink:type="arc" order="1"/>
  </link:definitionLink>
</link:linkbase>
