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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e42a3-b127-7750-9f6e-7be3a4d70b5b,g:725d73c2-3da9-4e35-8c8f-aea4300fc06e-->
<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-20260630.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_019e42a3-b275-7aeb-a589-edd5562178b6" 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_019e42a3-b275-709c-94ea-a87dda3da478" 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_019e42a3-b275-7aeb-a589-edd5562178b6" xlink:to="loc_us-gaap_StatementTable_019e42a3-b275-709c-94ea-a87dda3da478" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e42a3-b275-7c79-bd98-f79c1c3540e4" 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_019e42a3-b275-709c-94ea-a87dda3da478" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e42a3-b275-7c79-bd98-f79c1c3540e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e42a3-b275-7c79-bd98-f79c1c3540e4_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_019e42a3-b275-7c79-bd98-f79c1c3540e4" xlink:to="loc_us-gaap_EquityComponentDomain_019e42a3-b275-7c79-bd98-f79c1c3540e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e42a3-b275-75f3-a3d9-6aa846f91d43" 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_019e42a3-b275-7c79-bd98-f79c1c3540e4" xlink:to="loc_us-gaap_EquityComponentDomain_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e42a3-b275-7622-a38f-3ba527b9635b" 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_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:to="loc_us-gaap_CommonStockMember_019e42a3-b275-7622-a38f-3ba527b9635b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e42a3-b275-7d06-aeb2-9c964b91b107" 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_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e42a3-b275-7d06-aeb2-9c964b91b107" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e42a3-b275-7bea-ae4e-a0a4d141a794" 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_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e42a3-b275-7bea-ae4e-a0a4d141a794" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e42a3-b275-799a-9b4e-29875682515e" 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_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:to="loc_us-gaap_RetainedEarningsMember_019e42a3-b275-799a-9b4e-29875682515e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019e42a3-b275-7667-b94e-491f615c3b3d" 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_019e42a3-b275-75f3-a3d9-6aa846f91d43" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019e42a3-b275-7667-b94e-491f615c3b3d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e42a3-b275-7d88-821e-1720103be3f7" 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_019e42a3-b275-7aeb-a589-edd5562178b6" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e42a3-b275-7d88-821e-1720103be3f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e42a3-b275-761c-8c1a-2f8cc06e73c5" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e42a3-b275-761c-8c1a-2f8cc06e73c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e42a3-b275-73e7-a605-d237266679be" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_StockholdersEquity_019e42a3-b275-73e7-a605-d237266679be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019e42a3-b275-7ebd-8fbd-6f6daf9fcab4" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019e42a3-b275-7ebd-8fbd-6f6daf9fcab4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e42a3-b275-7827-a2bb-4fe1db6f875c" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e42a3-b275-7827-a2bb-4fe1db6f875c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_019e42a3-b275-7c2b-8d53-d78bc4753d52" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_019e42a3-b275-7c2b-8d53-d78bc4753d52" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019e42a3-b275-7e9c-99d8-0b6ce4cc65b6" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019e42a3-b275-7e9c-99d8-0b6ce4cc65b6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019fc469-3fd3-7d59-8623-058b045b1a1b" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019fc469-3fd3-7d59-8623-058b045b1a1b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019fc469-3fd3-7134-8794-a3f1d2649f4e" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019fc469-3fd3-7134-8794-a3f1d2649f4e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_019fc468-2884-76d7-bb5a-904602e411fa" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_019fc468-2884-76d7-bb5a-904602e411fa" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_019fc468-5b6d-7c46-8fe5-f9cb71c0c385" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_019fc468-5b6d-7c46-8fe5-f9cb71c0c385" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e42a3-b275-70c7-a4c9-77b5b976b0f5" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e42a3-b275-70c7-a4c9-77b5b976b0f5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e42a3-b275-7d36-bbd7-8ce5487eab17" 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_019e42a3-b275-7d88-821e-1720103be3f7" xlink:to="loc_us-gaap_ProfitLoss_019e42a3-b275-7d36-bbd7-8ce5487eab17" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e42a3-b275-771b-9f1c-03e3db183db9" 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_019e42a3-b275-7d09-86fb-820143288fed" 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-20260630.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_019fc470-a55a-7050-b777-ee4d8f731ef8" 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_019fc470-a55a-7b0d-81d0-cb1fb47c48b7" 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_019fc470-a55a-7050-b777-ee4d8f731ef8" xlink:to="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_019fc470-a55a-7b0d-81d0-cb1fb47c48b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019fc470-a55a-7eac-a310-d9d900281385" 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_019fc470-a55a-7b0d-81d0-cb1fb47c48b7" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019fc470-a55a-7eac-a310-d9d900281385" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7eac-a310-d9d900281385_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_019fc470-a55a-7eac-a310-d9d900281385" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7eac-a310-d9d900281385_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7941-9921-acf2e589a133" 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_019fc470-a55a-7eac-a310-d9d900281385" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7941-9921-acf2e589a133" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OrganizationalRestructuringMember_019fc470-a55a-7556-b4a6-d59e340f4b38" xlink:href="algt-20260630.xsd#algt_OrganizationalRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7941-9921-acf2e589a133" xlink:to="loc_algt_OrganizationalRestructuringMember_019fc470-a55a-7556-b4a6-d59e340f4b38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SaleOfSunseekerResortMember_019fc470-a55a-7917-887c-d49dfa8fe35b" xlink:href="algt-20260630.xsd#algt_SaleOfSunseekerResortMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019fc470-a55a-7941-9921-acf2e589a133" xlink:to="loc_algt_SaleOfSunseekerResortMember_019fc470-a55a-7917-887c-d49dfa8fe35b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfAircraftRetired_019fc4dd-200c-7bdd-916c-a2a2509c3af4" xlink:href="algt-20260630.xsd#algt_NumberOfAircraftRetired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_019fc470-a55a-7050-b777-ee4d8f731ef8" xlink:to="loc_algt_NumberOfAircraftRetired_019fc4dd-200c-7bdd-916c-a2a2509c3af4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019fc470-a55a-7c5e-b336-e44de53833ea" 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_019fc470-a55a-7050-b777-ee4d8f731ef8" xlink:to="loc_us-gaap_RestructuringCharges_019fc470-a55a-7c5e-b336-e44de53833ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfFlightSimulators_019fc470-a55a-77d7-89ba-faa72edca5db" xlink:href="algt-20260630.xsd#algt_NumberOfFlightSimulators"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_019fc470-a55a-7050-b777-ee4d8f731ef8" xlink:to="loc_algt_NumberOfFlightSimulators_019fc470-a55a-77d7-89ba-faa72edca5db" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SpecialChargesSummaryofSpecialChargesDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b275-7a4f-9b89-34577a684cd1" 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_019e42a3-b275-7c41-929f-38194af68640" 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_019e42a3-b275-7a4f-9b89-34577a684cd1" xlink:to="loc_us-gaap_ScheduleOfBusinessInsuranceRecoveriesTable_019e42a3-b275-7c41-929f-38194af68640" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019e42a3-b275-75aa-8be4-515938dbd92f" 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_019e42a3-b275-7c41-929f-38194af68640" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019e42a3-b275-75aa-8be4-515938dbd92f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-75aa-8be4-515938dbd92f_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_019e42a3-b275-75aa-8be4-515938dbd92f" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-75aa-8be4-515938dbd92f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" 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_019e42a3-b275-75aa-8be4-515938dbd92f" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember_019e42a3-b275-797b-b218-da66729be6cc" xlink:href="algt-20260630.xsd#algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_AcceleratedDepreciationOnAirframesIdentifiedForEarlyRetirementMember_019e42a3-b275-797b-b218-da66729be6cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember_019e42a3-b275-70d6-8b01-1bc80d274d64" xlink:href="algt-20260630.xsd#algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_AcceleratedAmortizationAndDisposalOfSoftwareIdentifiedForRedevelopmentMember_019e42a3-b275-70d6-8b01-1bc80d274d64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_IntegrationCostsMember_019e42a3-b275-7a68-a8a3-2da5924f4538" xlink:href="algt-20260630.xsd#algt_IntegrationCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_IntegrationCostsMember_019e42a3-b275-7a68-a8a3-2da5924f4538" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OrganizationalRestructuringMember_019fc470-e8d9-7cac-a642-6bc442bb8c9b" xlink:href="algt-20260630.xsd#algt_OrganizationalRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_OrganizationalRestructuringMember_019fc470-e8d9-7cac-a642-6bc442bb8c9b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditLossOnNoteReceivableMember_019e42a3-b275-7d03-8247-35d60a0c4917" xlink:href="algt-20260630.xsd#algt_CreditLossOnNoteReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_CreditLossOnNoteReceivableMember_019e42a3-b275-7d03-8247-35d60a0c4917" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunseekerSpecialChargesMember_019e42a3-b275-7451-965d-8735ac0687b4" xlink:href="algt-20260630.xsd#algt_SunseekerSpecialChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e42a3-b275-7893-933d-9f139007e368" xlink:to="loc_algt_SunseekerSpecialChargesMember_019e42a3-b275-7451-965d-8735ac0687b4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019fc472-24ca-7865-a986-e1fa895e920e" 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_019e42a3-b275-7c41-929f-38194af68640" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019fc472-24ca-7865-a986-e1fa895e920e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fc472-24ca-7865-a986-e1fa895e920e_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_019fc472-24ca-7865-a986-e1fa895e920e" xlink:to="loc_us-gaap_SegmentDomain_019fc472-24ca-7865-a986-e1fa895e920e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fc472-24ca-79d4-93cf-4fc7ddcea911" 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_019fc472-24ca-7865-a986-e1fa895e920e" xlink:to="loc_us-gaap_SegmentDomain_019fc472-24ca-79d4-93cf-4fc7ddcea911" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirlineMember_019fc472-24ca-70ad-8581-1cf17f56796a" xlink:href="algt-20260630.xsd#algt_AirlineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019fc472-24ca-79d4-93cf-4fc7ddcea911" xlink:to="loc_algt_AirlineMember_019fc472-24ca-70ad-8581-1cf17f56796a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019e42a3-b275-78df-b2b7-5a4de0cf54a0" 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_019e42a3-b275-7a4f-9b89-34577a684cd1" xlink:to="loc_us-gaap_RestructuringCharges_019e42a3-b275-78df-b2b7-5a4de0cf54a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RestructuringChargesNetOfRecoveries_019e42a3-b275-777a-acd9-f1fdfcc84156" xlink:href="algt-20260630.xsd#algt_RestructuringChargesNetOfRecoveries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessInterruptionLossLineItems_019e42a3-b275-7a4f-9b89-34577a684cd1" xlink:to="loc_algt_RestructuringChargesNetOfRecoveries_019e42a3-b275-777a-acd9-f1fdfcc84156" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_019e42a3-b275-7c89-b283-96fca299059f" 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_019e42a3-b275-7a4f-9b89-34577a684cd1" xlink:to="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_019e42a3-b275-7c89-b283-96fca299059f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/RevenueRecognitionDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b275-7397-93f9-20c7d50f5add" 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_019e42a3-b275-72b2-baad-10f8eccaa836" 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_019e42a3-b275-7397-93f9-20c7d50f5add" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e42a3-b275-72b2-baad-10f8eccaa836" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e42a3-b275-7138-baae-a2b8ca869b4c" 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_019e42a3-b275-72b2-baad-10f8eccaa836" xlink:to="loc_srt_ProductOrServiceAxis_019e42a3-b275-7138-baae-a2b8ca869b4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e42a3-b275-7138-baae-a2b8ca869b4c_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_019e42a3-b275-7138-baae-a2b8ca869b4c" xlink:to="loc_srt_ProductsAndServicesDomain_019e42a3-b275-7138-baae-a2b8ca869b4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" 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_019e42a3-b275-7138-baae-a2b8ca869b4c" xlink:to="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ScheduledServiceRevenueMember_019e42a3-b275-7e5f-bc67-f73a172533f9" xlink:href="algt-20260630.xsd#algt_ScheduledServiceRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_algt_ScheduledServiceRevenueMember_019e42a3-b275-7e5f-bc67-f73a172533f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirrelatedrevenueMember_019e42a3-b275-759c-be22-a9b2b2423155" xlink:href="algt-20260630.xsd#algt_AirrelatedrevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_algt_AirrelatedrevenueMember_019e42a3-b275-759c-be22-a9b2b2423155" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CobrandRevenueMember_019e42a3-b275-77e4-8abe-b7a6d49d2e1d" xlink:href="algt-20260630.xsd#algt_CobrandRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_algt_CobrandRevenueMember_019e42a3-b275-77e4-8abe-b7a6d49d2e1d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RoomsMember_019e42a3-b275-71b1-99f7-e8fa3dd67a50" xlink:href="algt-20260630.xsd#algt_RoomsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_algt_RoomsMember_019e42a3-b275-71b1-99f7-e8fa3dd67a50" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FoodAndBeverageMember_019e42a3-b275-71cf-a2ca-623d9aab6a7b" 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_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_us-gaap_FoodAndBeverageMember_019e42a3-b275-71cf-a2ca-623d9aab6a7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductAndServiceOtherMember_019e42a3-b275-79eb-8e06-e55f7d31f433" 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_019e42a3-b275-74ca-8333-fe03375516f6" xlink:to="loc_us-gaap_ProductAndServiceOtherMember_019e42a3-b275-79eb-8e06-e55f7d31f433" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e42a3-b275-7590-bf56-8bd40c6e3aee" 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_019e42a3-b275-72b2-baad-10f8eccaa836" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e42a3-b275-7590-bf56-8bd40c6e3aee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e42a3-b275-7590-bf56-8bd40c6e3aee_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_019e42a3-b275-7590-bf56-8bd40c6e3aee" xlink:to="loc_us-gaap_SegmentDomain_019e42a3-b275-7590-bf56-8bd40c6e3aee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e42a3-b275-7195-8a35-c8630616385f" 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_019e42a3-b275-7590-bf56-8bd40c6e3aee" xlink:to="loc_us-gaap_SegmentDomain_019e42a3-b275-7195-8a35-c8630616385f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PassengerRevenueMember_019e42a3-b275-76ed-8211-4382fa7c6bb0" xlink:href="algt-20260630.xsd#algt_PassengerRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e42a3-b275-7195-8a35-c8630616385f" xlink:to="loc_algt_PassengerRevenueMember_019e42a3-b275-76ed-8211-4382fa7c6bb0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ResortRevenueMember_019fc477-0bb5-70a7-a6dd-4ab27b101e91" xlink:href="algt-20260630.xsd#algt_ResortRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e42a3-b275-7195-8a35-c8630616385f" xlink:to="loc_algt_ResortRevenueMember_019fc477-0bb5-70a7-a6dd-4ab27b101e91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e42a3-b275-7244-9bfe-d8ae331afeff" 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_019e42a3-b275-7397-93f9-20c7d50f5add" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e42a3-b275-7244-9bfe-d8ae331afeff" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/RevenueRecognitionNarrativeDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b275-7d9c-8d79-571f77bde466" 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_019e42a3-b275-7fc4-ba04-840e880509d0" 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_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e42a3-b275-7fc4-ba04-840e880509d0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e42a3-b275-715f-b325-7783c4f86d53" 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_019e42a3-b275-7fc4-ba04-840e880509d0" xlink:to="loc_srt_ProductOrServiceAxis_019e42a3-b275-715f-b325-7783c4f86d53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e42a3-b275-715f-b325-7783c4f86d53_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_019e42a3-b275-715f-b325-7783c4f86d53" xlink:to="loc_srt_ProductsAndServicesDomain_019e42a3-b275-715f-b325-7783c4f86d53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e42a3-b275-799a-a03e-7e735270e3fb" 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_019e42a3-b275-715f-b325-7783c4f86d53" xlink:to="loc_srt_ProductsAndServicesDomain_019e42a3-b275-799a-a03e-7e735270e3fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ThirdPartyProductRevenueMember_019e42a3-b275-729f-b346-9124b8e03571" xlink:href="algt-20260630.xsd#algt_ThirdPartyProductRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e42a3-b275-799a-a03e-7e735270e3fb" xlink:to="loc_algt_ThirdPartyProductRevenueMember_019e42a3-b275-729f-b346-9124b8e03571" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e42a3-b275-76c9-a006-48547aaca3d3" 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_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e42a3-b275-76c9-a006-48547aaca3d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ContractwithCustomerLiabilityForwardBookings_019e42a3-b275-7c77-b110-085f3f94ce82" xlink:href="algt-20260630.xsd#algt_ContractwithCustomerLiabilityForwardBookings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_algt_ContractwithCustomerLiabilityForwardBookings_019e42a3-b275-7c77-b110-085f3f94ce82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ContractwithCustomerLiabilityCreditVoucherBookings_019e42a3-b275-7887-8ad1-a58179ba5822" xlink:href="algt-20260630.xsd#algt_ContractwithCustomerLiabilityCreditVoucherBookings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_algt_ContractwithCustomerLiabilityCreditVoucherBookings_019e42a3-b275-7887-8ad1-a58179ba5822" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e42a3-b275-72e4-933e-76be925daa5e" 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_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e42a3-b275-72e4-933e-76be925daa5e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirTrafficLiabilityRecognized_019e42a3-b275-78f6-b4b5-727e76c2d88d" xlink:href="algt-20260630.xsd#algt_AirTrafficLiabilityRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_algt_AirTrafficLiabilityRecognized_019e42a3-b275-78f6-b4b5-727e76c2d88d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueFromContractWithCustomerConcentrationPercentage_019e42a3-b275-73b0-a129-5bbff0a416d8" xlink:href="algt-20260630.xsd#algt_RevenueFromContractWithCustomerConcentrationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_algt_RevenueFromContractWithCustomerConcentrationPercentage_019e42a3-b275-73b0-a129-5bbff0a416d8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019e42a3-b275-7850-b97d-426dc6fbf1bd" 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_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_us-gaap_Revenues_019e42a3-b275-7850-b97d-426dc6fbf1bd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueContractAssetAmount_019fd89e-9d2d-77f8-ae24-1946a270f813" xlink:href="algt-20260630.xsd#algt_RevenueContractAssetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e42a3-b275-7d9c-8d79-571f77bde466" xlink:to="loc_algt_RevenueContractAssetAmount_019fd89e-9d2d-77f8-ae24-1946a270f813" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/PropertyandEquipmentDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b275-7429-86a8-90691314dc1e" 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_019e42a3-b275-781a-8af2-e476cf95f0bc" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e42a3-b275-781a-8af2-e476cf95f0bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e42a3-b275-72cb-b974-ad15eefc03ca" 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_019e42a3-b275-781a-8af2-e476cf95f0bc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e42a3-b275-72cb-b974-ad15eefc03ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e42a3-b275-72cb-b974-ad15eefc03ca_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_019e42a3-b275-72cb-b974-ad15eefc03ca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e42a3-b275-72cb-b974-ad15eefc03ca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e42a3-b275-7df5-b557-32a4b1798359" 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_019e42a3-b275-72cb-b974-ad15eefc03ca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e42a3-b275-7df5-b557-32a4b1798359" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AirlineMember_019e42a3-b275-7d3f-be30-5269337124cc" xlink:href="algt-20260630.xsd#algt_AirlineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e42a3-b275-7df5-b557-32a4b1798359" xlink:to="loc_algt_AirlineMember_019e42a3-b275-7d3f-be30-5269337124cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FlightEquipmentGross_019e42a3-b275-7abe-b49a-d347cb292b05" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_FlightEquipmentGross_019e42a3-b275-7abe-b49a-d347cb292b05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ComputerHardwareandSoftware_019e42a3-b275-710f-83de-ac8ddf9d726e" xlink:href="algt-20260630.xsd#algt_ComputerHardwareandSoftware"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_algt_ComputerHardwareandSoftware_019e42a3-b275-710f-83de-ac8ddf9d726e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_Landandbuildingsleaseholdimprovements_019e42a3-b275-7a45-8eb1-bc0c4dae3128" xlink:href="algt-20260630.xsd#algt_Landandbuildingsleaseholdimprovements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_algt_Landandbuildingsleaseholdimprovements_019e42a3-b275-7a45-8eb1-bc0c4dae3128" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentOther_019e42a3-b275-79f5-a7c2-5ee0cd42ddd7" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentOther_019e42a3-b275-79f5-a7c2-5ee0cd42ddd7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019e42a3-b275-79e7-be5e-cf3a453f5cef" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019e42a3-b275-79e7-be5e-cf3a453f5cef" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e42a3-b275-7220-aa90-f276200d2c48" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e42a3-b275-7220-aa90-f276200d2c48" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e42a3-b275-7d7b-b140-6640b8d20f1b" 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_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e42a3-b275-7d7b-b140-6640b8d20f1b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfAircraftCommittedToPurchase_019e42a3-b275-7111-87fa-39f2d048a36f" xlink:href="algt-20260630.xsd#algt_NumberOfAircraftCommittedToPurchase"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e42a3-b275-7429-86a8-90691314dc1e" xlink:to="loc_algt_NumberOfAircraftCommittedToPurchase_019e42a3-b275-7111-87fa-39f2d048a36f" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/LongTermDebtComponentsofLongTermDebtDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b275-7fdb-aeae-cf2f942415f1" 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_019e42a3-b275-7227-88a6-5ba05a305071" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_DebtInstrumentTable_019e42a3-b275-7227-88a6-5ba05a305071" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e42a3-b275-7da5-abbd-7501e974ad0b" 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_019e42a3-b275-7227-88a6-5ba05a305071" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e42a3-b275-7da5-abbd-7501e974ad0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7da5-abbd-7501e974ad0b_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_019e42a3-b275-7da5-abbd-7501e974ad0b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7da5-abbd-7501e974ad0b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" 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_019e42a3-b275-7da5-abbd-7501e974ad0b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_FixedRateMember_019e42a3-b275-7dab-b068-04cb01e94503" xlink:href="algt-20260630.xsd#algt_FixedRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_FixedRateMember_019e42a3-b275-7dab-b068-04cb01e94503" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_VariableRateMember_019e42a3-b275-716e-a7aa-3f8e6933d7db" xlink:href="algt-20260630.xsd#algt_VariableRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_VariableRateMember_019e42a3-b275-716e-a7aa-3f8e6933d7db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredNotesDue2027Member_019e42a3-b275-797c-817a-890158175dfd" xlink:href="algt-20260630.xsd#algt_SeniorSecuredNotesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_SeniorSecuredNotesDue2027Member_019e42a3-b275-797c-817a-890158175dfd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ConsolidatedVariableInterestEntitiesDue2029Member_019e42a3-b275-70fa-b285-da793710c69e" xlink:href="algt-20260630.xsd#algt_ConsolidatedVariableInterestEntitiesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_ConsolidatedVariableInterestEntitiesDue2029Member_019e42a3-b275-70fa-b285-da793710c69e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevolvingCreditFacilitiesDue2027Member_019e42a3-b275-73cd-86c0-14c8d1bb7282" xlink:href="algt-20260630.xsd#algt_RevolvingCreditFacilitiesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_RevolvingCreditFacilitiesDue2027Member_019e42a3-b275-73cd-86c0-14c8d1bb7282" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember_019e42a3-b275-730a-960e-11440feb5122" xlink:href="algt-20260630.xsd#algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_algt_SecuredDebtByAircraftEnginesOtherEquipmentAndRealEstateMember_019e42a3-b275-730a-960e-11440feb5122" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMember_019e42a3-b275-7345-87bb-33d3ca3f6341" 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_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_us-gaap_LongTermDebtMember_019e42a3-b275-7345-87bb-33d3ca3f6341" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnhancedEquipmentTrustCertificateMember_019fc4e3-4f37-7829-aedd-d19b0e5cb4d9" 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_019e42a3-b275-7c44-915f-e29caaddd855" xlink:to="loc_us-gaap_EnhancedEquipmentTrustCertificateMember_019fc4e3-4f37-7829-aedd-d19b0e5cb4d9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e42a3-b275-781a-b402-2378a60c4c3e" 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_019e42a3-b275-7227-88a6-5ba05a305071" xlink:to="loc_srt_RangeAxis_019e42a3-b275-781a-b402-2378a60c4c3e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e42a3-b275-781a-b402-2378a60c4c3e_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_019e42a3-b275-781a-b402-2378a60c4c3e" xlink:to="loc_srt_RangeMember_019e42a3-b275-781a-b402-2378a60c4c3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e42a3-b275-7c1c-868e-3bad380dfaa7" 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_019e42a3-b275-781a-b402-2378a60c4c3e" xlink:to="loc_srt_RangeMember_019e42a3-b275-7c1c-868e-3bad380dfaa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e42a3-b275-751f-8e91-59f6bcb89cca" 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_019e42a3-b275-7c1c-868e-3bad380dfaa7" xlink:to="loc_srt_MinimumMember_019e42a3-b275-751f-8e91-59f6bcb89cca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e42a3-b275-729f-9058-2e430177520d" 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_019e42a3-b275-7c1c-868e-3bad380dfaa7" xlink:to="loc_srt_MaximumMember_019e42a3-b275-729f-9058-2e430177520d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e42a3-b275-7afb-a8d4-f4b22755aa08" 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_019e42a3-b275-7227-88a6-5ba05a305071" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e42a3-b275-7afb-a8d4-f4b22755aa08" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b275-7afb-a8d4-f4b22755aa08_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_019e42a3-b275-7afb-a8d4-f4b22755aa08" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b275-7afb-a8d4-f4b22755aa08_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b275-7b7a-9e49-a6f9daed9427" 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_019e42a3-b275-7afb-a8d4-f4b22755aa08" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b275-7b7a-9e49-a6f9daed9427" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019e42a3-b275-7f7e-8f1e-baf09a759194" 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_019e42a3-b275-7b7a-9e49-a6f9daed9427" xlink:to="loc_us-gaap_SeniorNotesMember_019e42a3-b275-7f7e-8f1e-baf09a759194" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e42a3-b275-77eb-9913-d2f713a852da" 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_019e42a3-b275-7b7a-9e49-a6f9daed9427" xlink:to="loc_us-gaap_SecuredDebtMember_019e42a3-b275-77eb-9913-d2f713a852da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e42a3-b275-7d07-ba74-4944ff22a022" 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_019e42a3-b275-7b7a-9e49-a6f9daed9427" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e42a3-b275-7d07-ba74-4944ff22a022" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019e42a3-b275-778d-9238-ec2bf9cab339" 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_019e42a3-b275-7b7a-9e49-a6f9daed9427" xlink:to="loc_us-gaap_UnsecuredDebtMember_019e42a3-b275-778d-9238-ec2bf9cab339" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayable_019e42a3-b275-7ccb-801d-5059c822de69" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_NotesPayable_019e42a3-b275-7ccb-801d-5059c822de69" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredLongTermDebt_019e42a3-b275-7dbb-ac70-e75ea52b04fb" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_SecuredLongTermDebt_019e42a3-b275-7dbb-ac70-e75ea52b04fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e42a3-b275-7e67-a345-b3b0c03ffcd8" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e42a3-b275-7e67-a345-b3b0c03ffcd8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e42a3-b275-7609-96bc-15a0233683a7" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e42a3-b275-7609-96bc-15a0233683a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019e42a3-b275-72e8-8547-117e33caa6f5" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019e42a3-b275-72e8-8547-117e33caa6f5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e42a3-b275-7117-83b6-d2d4c1ff49c4" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e42a3-b275-7117-83b6-d2d4c1ff49c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019e42a3-b275-7f0d-8608-0712287e075a" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_FinanceLeaseLiability_019e42a3-b275-7f0d-8608-0712287e075a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_019e42a3-b275-7f14-a996-1aa96f84e94a" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_019e42a3-b275-7f14-a996-1aa96f84e94a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019e42a3-b275-7ecb-8929-34d78d3d3e91" 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_019e42a3-b275-7fdb-aeae-cf2f942415f1" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019e42a3-b275-7ecb-8929-34d78d3d3e91" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="algt-20260630.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_019fc4a9-591b-72ef-b234-82fbda90a653" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentTable_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fc4a9-591b-7bea-9aec-a9c6244853fd" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fc4a9-591b-7bea-9aec-a9c6244853fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc4a9-591b-7bea-9aec-a9c6244853fd_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_019fc4a9-591b-7bea-9aec-a9c6244853fd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc4a9-591b-7bea-9aec-a9c6244853fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc4a9-591b-7ddc-acb6-8cd7be54b32c" 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_019fc4a9-591b-7bea-9aec-a9c6244853fd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc4a9-591b-7ddc-acb6-8cd7be54b32c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019fc4a9-591b-72de-a9fc-950181d57f3f" 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_019fc4a9-591b-7ddc-acb6-8cd7be54b32c" xlink:to="loc_us-gaap_SecuredDebtMember_019fc4a9-591b-72de-a9fc-950181d57f3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019fc4a9-87d4-7f6f-9901-84f0adc7e638" 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_019fc4a9-591b-7ddc-acb6-8cd7be54b32c" xlink:to="loc_us-gaap_LineOfCreditMember_019fc4a9-87d4-7f6f-9901-84f0adc7e638" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019fc4a9-d627-7e82-954b-5be19334ca82" 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_019fc4a9-591b-7ddc-acb6-8cd7be54b32c" xlink:to="loc_us-gaap_SeniorNotesMember_019fc4a9-d627-7e82-954b-5be19334ca82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019fc4a9-87d4-70a6-9adb-f563ff982c5e" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_CreditFacilityAxis_019fc4a9-87d4-70a6-9adb-f563ff982c5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc4a9-87d4-70a6-9adb-f563ff982c5e_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_019fc4a9-87d4-70a6-9adb-f563ff982c5e" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc4a9-87d4-70a6-9adb-f563ff982c5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc4a9-87d4-7b74-9cf1-6ae8e4587965" 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_019fc4a9-87d4-70a6-9adb-f563ff982c5e" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc4a9-87d4-7b74-9cf1-6ae8e4587965" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019fc4a9-87d4-7e59-9647-f996c01ef391" 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_019fc4a9-87d4-7b74-9cf1-6ae8e4587965" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019fc4a9-87d4-7e59-9647-f996c01ef391" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredRevolvingLoanFacilityMember_019fc4a9-fd86-75e0-9003-a56cdaa77207" xlink:href="algt-20260630.xsd#algt_SeniorSecuredRevolvingLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019fc4a9-87d4-7b74-9cf1-6ae8e4587965" xlink:to="loc_algt_SeniorSecuredRevolvingLoanFacilityMember_019fc4a9-fd86-75e0-9003-a56cdaa77207" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fc4a9-87d4-71d6-b634-d394adf69bce" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fc4a9-87d4-71d6-b634-d394adf69bce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-71d6-b634-d394adf69bce_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_019fc4a9-87d4-71d6-b634-d394adf69bce" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-71d6-b634-d394adf69bce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" 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_019fc4a9-87d4-71d6-b634-d394adf69bce" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementCANSBCANSB1Member_019fc4a9-87d4-70cc-9412-d5b9fe2d474b" xlink:href="algt-20260630.xsd#algt_CreditAgreementCANSBCANSB1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_CreditAgreementCANSBCANSB1Member_019fc4a9-87d4-70cc-9412-d5b9fe2d474b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementSMTB3Member_019fc4a9-87d4-7691-b2d1-069152f6a427" xlink:href="algt-20260630.xsd#algt_CreditAgreementSMTB3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_CreditAgreementSMTB3Member_019fc4a9-87d4-7691-b2d1-069152f6a427" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementDBJ2DBJ21Member_019fc4a9-87d4-773a-922b-336b5a872155" xlink:href="algt-20260630.xsd#algt_CreditAgreementDBJ2DBJ21Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_CreditAgreementDBJ2DBJ21Member_019fc4a9-87d4-773a-922b-336b5a872155" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredNotesDue2031Member_019fc4a9-d627-717b-8a59-fcf29d220afd" xlink:href="algt-20260630.xsd#algt_SeniorSecuredNotesDue2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_SeniorSecuredNotesDue2031Member_019fc4a9-d627-717b-8a59-fcf29d220afd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SeniorSecuredNotesDue2027Member_019fc4a9-d627-757d-94e8-9fbe31fc9385" xlink:href="algt-20260630.xsd#algt_SeniorSecuredNotesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_SeniorSecuredNotesDue2027Member_019fc4a9-d627-757d-94e8-9fbe31fc9385" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OtherSecuredDebtMember_019fc4a9-d627-7762-82f7-8f3de4f0d683" xlink:href="algt-20260630.xsd#algt_OtherSecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4a9-87d4-793a-bb0d-1e1805568867" xlink:to="loc_algt_OtherSecuredDebtMember_019fc4a9-d627-7762-82f7-8f3de4f0d683" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0" 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_DebtInstrumentTable_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0_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_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4a9-a5c1-75e0-9c0e-0f4ee7e24265" 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_019fc4a9-a5c1-7f25-8b17-64a108fd8cc0" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4a9-a5c1-75e0-9c0e-0f4ee7e24265" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryAirlinesMember_019fc4a9-a5c1-7205-88c4-8ecb73cc50cd" xlink:href="algt-20260630.xsd#algt_SunCountryAirlinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4a9-a5c1-75e0-9c0e-0f4ee7e24265" xlink:to="loc_algt_SunCountryAirlinesMember_019fc4a9-a5c1-7205-88c4-8ecb73cc50cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019fc4a9-a5c1-776c-9e3b-8809021e4070" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_srt_RangeAxis_019fc4a9-a5c1-776c-9e3b-8809021e4070" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fc4a9-a5c1-776c-9e3b-8809021e4070_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_019fc4a9-a5c1-776c-9e3b-8809021e4070" xlink:to="loc_srt_RangeMember_019fc4a9-a5c1-776c-9e3b-8809021e4070_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fc4a9-a5c1-7af2-962f-c4fcfc9ee239" 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_019fc4a9-a5c1-776c-9e3b-8809021e4070" xlink:to="loc_srt_RangeMember_019fc4a9-a5c1-7af2-962f-c4fcfc9ee239" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019fc4a9-a5c1-79c4-8590-9d0572758b91" 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_019fc4a9-a5c1-7af2-962f-c4fcfc9ee239" xlink:to="loc_srt_MinimumMember_019fc4a9-a5c1-79c4-8590-9d0572758b91" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019fc4a9-a5c1-708c-a2c1-4f7750ea6f84" 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_019fc4a9-a5c1-7af2-962f-c4fcfc9ee239" xlink:to="loc_srt_MaximumMember_019fc4a9-a5c1-708c-a2c1-4f7750ea6f84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fc4a9-d627-7819-88c7-9e776b9d2689" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fc4a9-d627-7819-88c7-9e776b9d2689" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fc4a9-d627-7819-88c7-9e776b9d2689_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fc4a9-d627-7819-88c7-9e776b9d2689" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fc4a9-d627-7819-88c7-9e776b9d2689_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fc4a9-d627-71bd-b1cc-b26f147c34a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fc4a9-d627-7819-88c7-9e776b9d2689" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fc4a9-d627-71bd-b1cc-b26f147c34a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fc4a9-d627-7202-baee-d2a4f59633c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fc4a9-d627-71bd-b1cc-b26f147c34a0" xlink:to="loc_us-gaap_SubsequentEventMember_019fc4a9-d627-7202-baee-d2a4f59633c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0" 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_019e42a3-b275-7b25-903d-7eeb5f90f118" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0_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_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019fc4a9-fd86-75ed-9d82-37e73f2546fc" 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_019fc4a9-fd86-7e5a-8a3b-9ae61031c9b0" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019fc4a9-fd86-75ed-9d82-37e73f2546fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BarclaysBankPLCMember_019fc4a9-fd86-7641-b240-85a3e76c2503" xlink:href="algt-20260630.xsd#algt_BarclaysBankPLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_019fc4a9-fd86-75ed-9d82-37e73f2546fc" xlink:to="loc_algt_BarclaysBankPLCMember_019fc4a9-fd86-7641-b240-85a3e76c2503" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019fc4a9-591b-7005-ae01-b926c3d4f238" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019fc4a9-591b-7005-ae01-b926c3d4f238" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc4a9-87d4-77a7-843d-2b64e053eea8" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc4a9-87d4-77a7-843d-2b64e053eea8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019fc4a9-87d4-7e2c-b83d-9a7f989705f3" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_LineOfCredit_019fc4a9-87d4-7e2c-b83d-9a7f989705f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019fc4a9-87d4-74f3-9a0b-7f42abbea3e5" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019fc4a9-87d4-74f3-9a0b-7f42abbea3e5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019fc4a9-87d4-7229-9e39-e36b48c015e4" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentTerm_019fc4a9-87d4-7229-9e39-e36b48c015e4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_019fc4a9-a5c1-7ca0-9a74-e97677ebbec2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_019fc4a9-a5c1-7ca0-9a74-e97677ebbec2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfBoeingAircrafts_019fc4a9-a5c1-7237-8e15-d12138337522" xlink:href="algt-20260630.xsd#algt_NumberOfBoeingAircrafts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_NumberOfBoeingAircrafts_019fc4a9-a5c1-7237-8e15-d12138337522" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fc4a9-a5c1-73aa-adab-daee7cd97124" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fc4a9-a5c1-73aa-adab-daee7cd97124" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfAircraft_019fc4a9-a5c1-76e6-a8e5-99e3b54f9f3d" xlink:href="algt-20260630.xsd#algt_NumberOfAircraft"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_NumberOfAircraft_019fc4a9-a5c1-76e6-a8e5-99e3b54f9f3d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleLiquidationPreferenceValue_019fc4a9-d627-7a58-9bd9-60f1981ec422" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleLiquidationPreferenceValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentConvertibleLiquidationPreferenceValue_019fc4a9-d627-7a58-9bd9-60f1981ec422" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_DebtInstrumentConvertibleLiquidationPreferencePercentage_019fc4a9-d627-79d5-80dc-4971e2308a32" xlink:href="algt-20260630.xsd#algt_DebtInstrumentConvertibleLiquidationPreferencePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_DebtInstrumentConvertibleLiquidationPreferencePercentage_019fc4a9-d627-79d5-80dc-4971e2308a32" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_DebtInstrumentRedemptionAmount_019fc4a9-d627-72be-b373-b04339f63a3e" xlink:href="algt-20260630.xsd#algt_DebtInstrumentRedemptionAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_DebtInstrumentRedemptionAmount_019fc4a9-d627-72be-b373-b04339f63a3e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019fc4a9-d627-7a19-aa90-549a2ede3b9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_019fc4a9-d627-7a19-aa90-549a2ede3b9c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_019fc4a9-d627-7148-b65e-498dfa0de406" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_019fc4a9-d627-7148-b65e-498dfa0de406" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_019fc4a9-fd86-74c2-9e73-bcde963cf150" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_019fc4a9-fd86-74c2-9e73-bcde963cf150" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_TermLoanFacilityFairValue_019fd48e-b608-7cd2-a12e-5d6a3bf36faf" xlink:href="algt-20260630.xsd#algt_TermLoanFacilityFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_TermLoanFacilityFairValue_019fd48e-b608-7cd2-a12e-5d6a3bf36faf" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019fd8a4-b8f0-75c6-af2d-1a85234899aa" 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_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_us-gaap_FinanceLeaseLiability_019fd8a4-b8f0-75c6-af2d-1a85234899aa" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_EquipmentNotesOutstandingFairValueAmount_019fd8a6-84e1-719a-9ad5-dbc0d98089ba" xlink:href="algt-20260630.xsd#algt_EquipmentNotesOutstandingFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fc4a9-591b-72ef-b234-82fbda90a653" xlink:to="loc_algt_EquipmentNotesOutstandingFairValueAmount_019fd8a6-84e1-719a-9ad5-dbc0d98089ba" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b276-7fe1-b3ec-3603a6835c61" 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_019e42a3-b276-76ad-b846-63b35dd069bd" 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_019e42a3-b276-7fe1-b3ec-3603a6835c61" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e42a3-b276-76ad-b846-63b35dd069bd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e42a3-b276-7f28-be74-f81e8498484a" 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_019e42a3-b276-76ad-b846-63b35dd069bd" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e42a3-b276-7f28-be74-f81e8498484a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e42a3-b276-7f28-be74-f81e8498484a_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_019e42a3-b276-7f28-be74-f81e8498484a" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e42a3-b276-7f28-be74-f81e8498484a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e42a3-b276-76d4-a503-71969c06e255" 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_019e42a3-b276-7f28-be74-f81e8498484a" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e42a3-b276-76d4-a503-71969c06e255" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e42a3-b276-7fa5-8cd3-cbb193cf5657" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e42a3-b276-7fa5-8cd3-cbb193cf5657" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e42a3-b276-7b78-8153-bf718c5fc7cf" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_CommercialPaperMember_019e42a3-b276-7b78-8153-bf718c5fc7cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019e42a3-b276-719c-9369-a2ba92ec12e3" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019e42a3-b276-719c-9369-a2ba92ec12e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryBondSecuritiesMember_019e42a3-b276-7049-97a3-f3d8b45e062d" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_USTreasuryBondSecuritiesMember_019e42a3-b276-7049-97a3-f3d8b45e062d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e42a3-b276-7081-ab69-e58df06838e5" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e42a3-b276-7081-ab69-e58df06838e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_019e42a3-b276-77f0-8b47-e31825ba6a8f" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_019e42a3-b276-77f0-8b47-e31825ba6a8f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e42a3-b276-7410-9d74-8c85ebd27edf" 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_019e42a3-b276-76d4-a503-71969c06e255" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e42a3-b276-7410-9d74-8c85ebd27edf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e42a3-b276-7bc0-b221-0357668c7b48" 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_019e42a3-b276-76ad-b846-63b35dd069bd" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e42a3-b276-7bc0-b221-0357668c7b48" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7bc0-b221-0357668c7b48_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_019e42a3-b276-7bc0-b221-0357668c7b48" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7bc0-b221-0357668c7b48_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7468-98b1-cdb09e89e6c5" 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_019e42a3-b276-7bc0-b221-0357668c7b48" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7468-98b1-cdb09e89e6c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019e42a3-b276-76b2-9a73-20d32f7ba42b" 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_019e42a3-b276-7468-98b1-cdb09e89e6c5" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019e42a3-b276-76b2-9a73-20d32f7ba42b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e42a3-b276-78af-a09b-e44fb8b9ff49" 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_019e42a3-b276-76ad-b846-63b35dd069bd" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e42a3-b276-78af-a09b-e44fb8b9ff49" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e42a3-b276-78af-a09b-e44fb8b9ff49_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_019e42a3-b276-78af-a09b-e44fb8b9ff49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e42a3-b276-78af-a09b-e44fb8b9ff49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e42a3-b276-7786-8074-3c99a704ac1d" 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_019e42a3-b276-78af-a09b-e44fb8b9ff49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e42a3-b276-7786-8074-3c99a704ac1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_019e42a3-b276-7184-b02a-b8e9d1cdbe17" 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_019e42a3-b276-7786-8074-3c99a704ac1d" xlink:to="loc_us-gaap_CashEquivalentsMember_019e42a3-b276-7184-b02a-b8e9d1cdbe17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019e42a3-b276-70f6-b38d-902847056d3a" 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_019e42a3-b276-7786-8074-3c99a704ac1d" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019e42a3-b276-70f6-b38d-902847056d3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_LongTermInvestmentsMember_019e42a3-b276-7a9d-b9a2-76b539eebf62" xlink:href="algt-20260630.xsd#algt_LongTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e42a3-b276-7786-8074-3c99a704ac1d" xlink:to="loc_algt_LongTermInvestmentsMember_019e42a3-b276-7a9d-b9a2-76b539eebf62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e42a3-b276-7f4e-a00b-368f32a40987" 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_019e42a3-b276-76ad-b846-63b35dd069bd" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e42a3-b276-7f4e-a00b-368f32a40987" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-7f4e-a00b-368f32a40987_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_019e42a3-b276-7f4e-a00b-368f32a40987" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-7f4e-a00b-368f32a40987_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-7a29-99f4-98030e77f017" 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_019e42a3-b276-7f4e-a00b-368f32a40987" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-7a29-99f4-98030e77f017" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e42a3-b276-798e-adb7-44a318d40c01" 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_019e42a3-b276-7a29-99f4-98030e77f017" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e42a3-b276-798e-adb7-44a318d40c01" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e42a3-b276-73d8-b884-96804e9fa817" 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_019e42a3-b276-7a29-99f4-98030e77f017" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e42a3-b276-73d8-b884-96804e9fa817" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e42a3-b276-70e5-adc9-ced1b9c808e5" 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_019e42a3-b276-7fe1-b3ec-3603a6835c61" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e42a3-b276-70e5-adc9-ced1b9c808e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e42a3-b276-7c81-846b-8e6642559449" 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_019e42a3-b276-7fe1-b3ec-3603a6835c61" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e42a3-b276-7c81-846b-8e6642559449" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/FairValueMeasurementsEstimatedFairValueofDebtDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b276-7e13-af72-93ef716e6363" 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_019e42a3-b276-7400-821b-0d4b95a81519" 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_019e42a3-b276-7e13-af72-93ef716e6363" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e42a3-b276-7400-821b-0d4b95a81519" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e42a3-b276-72ab-8a98-fa06a730ab7f" 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_019e42a3-b276-7400-821b-0d4b95a81519" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e42a3-b276-72ab-8a98-fa06a730ab7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b276-72ab-8a98-fa06a730ab7f_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_019e42a3-b276-72ab-8a98-fa06a730ab7f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b276-72ab-8a98-fa06a730ab7f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b276-7e23-87b7-cebbe0539f2a" 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_019e42a3-b276-72ab-8a98-fa06a730ab7f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b276-7e23-87b7-cebbe0539f2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NonPubliclyHeldDebtMember_019e42a3-b276-7bb6-be24-a86b13e702a8" xlink:href="algt-20260630.xsd#algt_NonPubliclyHeldDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e42a3-b276-7e23-87b7-cebbe0539f2a" xlink:to="loc_algt_NonPubliclyHeldDebtMember_019e42a3-b276-7bb6-be24-a86b13e702a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e42a3-b276-788c-9e51-18865398841f" 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_019e42a3-b276-7400-821b-0d4b95a81519" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e42a3-b276-788c-9e51-18865398841f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-788c-9e51-18865398841f_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_019e42a3-b276-788c-9e51-18865398841f" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-788c-9e51-18865398841f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-781a-99de-58c1327abaf9" 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_019e42a3-b276-788c-9e51-18865398841f" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e42a3-b276-781a-99de-58c1327abaf9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e42a3-b276-7a9d-b5c7-377f0625c0c4" 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_019e42a3-b276-781a-99de-58c1327abaf9" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e42a3-b276-7a9d-b5c7-377f0625c0c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e42a3-b276-71e9-aa7e-fb9b71978d8c" 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_019e42a3-b276-7400-821b-0d4b95a81519" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e42a3-b276-71e9-aa7e-fb9b71978d8c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-71e9-aa7e-fb9b71978d8c_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_019e42a3-b276-71e9-aa7e-fb9b71978d8c" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-71e9-aa7e-fb9b71978d8c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7418-b9e8-1badefe567bf" 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_019e42a3-b276-71e9-aa7e-fb9b71978d8c" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e42a3-b276-7418-b9e8-1badefe567bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019e42a3-b276-73e6-91bd-b91ba0cfbcca" 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_019e42a3-b276-7418-b9e8-1badefe567bf" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019e42a3-b276-73e6-91bd-b91ba0cfbcca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredLongTermDebt_019e42a3-b276-76e5-a0e0-201fbe45f10f" 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_019e42a3-b276-7e13-af72-93ef716e6363" xlink:to="loc_us-gaap_SecuredLongTermDebt_019e42a3-b276-76e5-a0e0-201fbe45f10f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019e42a3-b276-7c3e-8d67-3f88a85a1ad6" 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_019e42a3-b276-7e13-af72-93ef716e6363" xlink:to="loc_us-gaap_LongTermDebtFairValue_019e42a3-b276-7c3e-8d67-3f88a85a1ad6" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SegmentsScheduleofSegmentReportingInformationbySegmentDetails" xlink:type="simple" xlink:href="algt-20260630.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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" 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_019e42a3-b276-7877-95bd-9aede184da57" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e42a3-b276-7877-95bd-9aede184da57" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6" 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_019e42a3-b276-7877-95bd-9aede184da57" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6_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_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6" xlink:to="loc_us-gaap_SegmentDomain_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e42a3-b276-72a7-ab46-bfe5c23ad4a9" 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_019e42a3-b276-7e39-8eb9-eeaeb54cf1a6" xlink:to="loc_us-gaap_SegmentDomain_019e42a3-b276-72a7-ab46-bfe5c23ad4a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AllegiantAirMember_019e42a3-b276-77fb-a1ee-f29f70f128d1" xlink:href="algt-20260630.xsd#algt_AllegiantAirMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e42a3-b276-72a7-ab46-bfe5c23ad4a9" xlink:to="loc_algt_AllegiantAirMember_019e42a3-b276-77fb-a1ee-f29f70f128d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryAirlinesMember_019fc4b1-efeb-7c0b-8509-0751e5a4ada3" xlink:href="algt-20260630.xsd#algt_SunCountryAirlinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e42a3-b276-72a7-ab46-bfe5c23ad4a9" xlink:to="loc_algt_SunCountryAirlinesMember_019fc4b1-efeb-7c0b-8509-0751e5a4ada3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunseekerResortsMember_019e42a3-b276-73b7-a74f-edf3d06b94c9" xlink:href="algt-20260630.xsd#algt_SunseekerResortsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e42a3-b276-72a7-ab46-bfe5c23ad4a9" xlink:to="loc_algt_SunseekerResortsMember_019e42a3-b276-73b7-a74f-edf3d06b94c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeAxis_019fd49e-adc7-7f4d-bf1e-49feebbc24f1" xlink:href="algt-20260630.xsd#algt_RevenueTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e42a3-b276-7877-95bd-9aede184da57" xlink:to="loc_algt_RevenueTypeAxis_019fd49e-adc7-7f4d-bf1e-49feebbc24f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeDomain_019fd49e-adc7-7f4d-bf1e-49feebbc24f1_default" xlink:href="algt-20260630.xsd#algt_RevenueTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_algt_RevenueTypeAxis_019fd49e-adc7-7f4d-bf1e-49feebbc24f1" xlink:to="loc_algt_RevenueTypeDomain_019fd49e-adc7-7f4d-bf1e-49feebbc24f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_RevenueTypeDomain_019fd49e-adc7-7ea3-87af-114aa342b0ae" xlink:href="algt-20260630.xsd#algt_RevenueTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_algt_RevenueTypeAxis_019fd49e-adc7-7f4d-bf1e-49feebbc24f1" xlink:to="loc_algt_RevenueTypeDomain_019fd49e-adc7-7ea3-87af-114aa342b0ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ScheduledServiceMember_019fd497-ec7f-71f7-a6ca-0950992e307f" xlink:href="algt-20260630.xsd#algt_ScheduledServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_019fd49e-adc7-7ea3-87af-114aa342b0ae" xlink:to="loc_algt_ScheduledServiceMember_019fd497-ec7f-71f7-a6ca-0950992e307f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AncillaryAirRelatedChargesMember_019fd498-3091-7f8a-8326-5848d2704e52" xlink:href="algt-20260630.xsd#algt_AncillaryAirRelatedChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_019fd49e-adc7-7ea3-87af-114aa342b0ae" xlink:to="loc_algt_AncillaryAirRelatedChargesMember_019fd498-3091-7f8a-8326-5848d2704e52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_LoyaltyRedemptionsMember_019fd498-7d61-7d4a-9d35-cc775d1c77de" xlink:href="algt-20260630.xsd#algt_LoyaltyRedemptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_algt_RevenueTypeDomain_019fd49e-adc7-7ea3-87af-114aa342b0ae" xlink:to="loc_algt_LoyaltyRedemptionsMember_019fd498-7d61-7d4a-9d35-cc775d1c77de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019fd897-44fc-725c-afce-1ac432ff81c3" 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_019e42a3-b276-7877-95bd-9aede184da57" xlink:to="loc_srt_ConsolidationItemsAxis_019fd897-44fc-725c-afce-1ac432ff81c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019fd897-44fc-725c-afce-1ac432ff81c3_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_019fd897-44fc-725c-afce-1ac432ff81c3" xlink:to="loc_srt_ConsolidationItemsDomain_019fd897-44fc-725c-afce-1ac432ff81c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019fd897-44fc-79ed-b2c9-6085fa3fe161" 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_019fd897-44fc-725c-afce-1ac432ff81c3" xlink:to="loc_srt_ConsolidationItemsDomain_019fd897-44fc-79ed-b2c9-6085fa3fe161" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019fd897-94d2-7711-bcb1-9405fc600ca9" 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_019fd897-44fc-79ed-b2c9-6085fa3fe161" xlink:to="loc_us-gaap_OperatingSegmentsMember_019fd897-94d2-7711-bcb1-9405fc600ca9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019fc4af-520b-7d4b-8805-a2e106bebdfa" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019fc4af-520b-7d4b-8805-a2e106bebdfa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ThirdPartyProducts_019fc4af-520b-7d45-b77b-38a6ef546db5" xlink:href="algt-20260630.xsd#algt_ThirdPartyProducts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_ThirdPartyProducts_019fc4af-520b-7d45-b77b-38a6ef546db5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_FixedFeeContractRevenue_019fc4af-520b-7e41-a8bb-20439b70e717" xlink:href="algt-20260630.xsd#algt_FixedFeeContractRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_FixedFeeContractRevenue_019fc4af-520b-7e41-a8bb-20439b70e717" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CargoRevenue_019fc4af-520b-76b1-9ea6-a88a924494ff" xlink:href="algt-20260630.xsd#algt_CargoRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_CargoRevenue_019fc4af-520b-76b1-9ea6-a88a924494ff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_OtherOperatingRevenue_019fc4af-520b-71fe-95b4-58da9fa8d17d" xlink:href="algt-20260630.xsd#algt_OtherOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_OtherOperatingRevenue_019fc4af-520b-71fe-95b4-58da9fa8d17d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019e42a3-b276-7b92-a1a8-0dd14dca3e85" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_Revenues_019e42a3-b276-7b92-a1a8-0dd14dca3e85" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_019e42a3-b276-716a-9b09-78e37f074a6b" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_LaborAndRelatedExpense_019e42a3-b276-716a-9b09-78e37f074a6b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AircraftFuel_019e42a3-b276-7b87-9c15-3a5fb862ef59" xlink:href="algt-20260630.xsd#algt_AircraftFuel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_AircraftFuel_019e42a3-b276-7b87-9c15-3a5fb862ef59" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_StationOperations_019e42a3-b276-7868-b6b1-134c8aa09d37" xlink:href="algt-20260630.xsd#algt_StationOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_StationOperations_019e42a3-b276-7868-b6b1-134c8aa09d37" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019e42a3-b276-7cea-94cc-f1fc981b255e" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_DepreciationAndAmortization_019e42a3-b276-7cea-94cc-f1fc981b255e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AircraftMaintenanceMaterialsAndRepairs_019e42a3-b276-7e82-b975-1e8304229a1c" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_AircraftMaintenanceMaterialsAndRepairs_019e42a3-b276-7e82-b975-1e8304229a1c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_019e42a3-b276-762f-a00d-a013420602b5" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_SellingAndMarketingExpense_019e42a3-b276-762f-a00d-a013420602b5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_AircraftLeaseRental_019e42a3-b276-78cf-ac01-8cf59d2769a0" xlink:href="algt-20260630.xsd#algt_AircraftLeaseRental"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_algt_AircraftLeaseRental_019e42a3-b276-78cf-ac01-8cf59d2769a0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostAndExpenseOperating_019e42a3-b276-7281-a4c1-b4229d05a81f" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_OtherCostAndExpenseOperating_019e42a3-b276-7281-a4c1-b4229d05a81f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_019e42a3-b276-76be-8dd3-916e6e412141" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_019e42a3-b276-76be-8dd3-916e6e412141" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_019e42a3-b276-76e0-b656-dabf35f259e6" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_OperatingExpenses_019e42a3-b276-76e0-b656-dabf35f259e6" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019e42a3-b276-7158-bbc2-5e1caf60e769" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_OperatingIncomeLoss_019e42a3-b276-7158-bbc2-5e1caf60e769" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNet_019e42a3-b276-70c8-8eac-5408f446a3bd" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_InterestIncomeExpenseNet_019e42a3-b276-70c8-8eac-5408f446a3bd" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_019e42a3-b276-71fc-950f-e04a955785b4" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_InterestExpense_019e42a3-b276-71fc-950f-e04a955785b4" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_019e42a3-b276-7d57-9658-04a5f5c77015" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_InterestCostsCapitalized_019e42a3-b276-7d57-9658-04a5f5c77015" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e42a3-b276-7221-bf15-c4525d5c8de0" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e42a3-b276-7221-bf15-c4525d5c8de0" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e42a3-b276-7899-aac6-76e0b43260d9" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e42a3-b276-7899-aac6-76e0b43260d9" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019fc4eb-3358-7a4c-9f3d-03e09c98a078" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019fc4eb-3358-7a4c-9f3d-03e09c98a078" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019fc4f0-53a4-73f6-abd9-fa236286d939" 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_019e42a3-b276-7ae9-8bc4-49552c15e7a8" xlink:to="loc_us-gaap_Assets_019fc4f0-53a4-73f6-abd9-fa236286d939" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncNarrativeDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" 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_019fc4c8-e22b-7da6-97b8-25553fe1facd" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fc4c8-e22b-7da6-97b8-25553fe1facd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fc4c8-e22b-74c5-abf0-3f80a0b54181" 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_019fc4c8-e22b-7da6-97b8-25553fe1facd" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fc4c8-e22b-74c5-abf0-3f80a0b54181" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-e22b-74c5-abf0-3f80a0b54181_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_019fc4c8-e22b-74c5-abf0-3f80a0b54181" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-e22b-74c5-abf0-3f80a0b54181_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-e22b-7c98-9a0a-ecdd5363f6ec" 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_019fc4c8-e22b-74c5-abf0-3f80a0b54181" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-e22b-7c98-9a0a-ecdd5363f6ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fc4c9-7bd4-734a-9d5b-6d6de5169372" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-e22b-7c98-9a0a-ecdd5363f6ec" xlink:to="loc_algt_SunCountryMember_019fc4c9-7bd4-734a-9d5b-6d6de5169372" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fd476-6736-7d53-9c50-77189b16e6d2" 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_019fc4c8-e22b-7da6-97b8-25553fe1facd" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fd476-6736-7d53-9c50-77189b16e6d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fd476-6736-7d53-9c50-77189b16e6d2_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_019fd476-6736-7d53-9c50-77189b16e6d2" xlink:to="loc_us-gaap_ClassOfStockDomain_019fd476-6736-7d53-9c50-77189b16e6d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fd476-6736-79c7-b31b-6885daa1407f" 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_019fd476-6736-7d53-9c50-77189b16e6d2" xlink:to="loc_us-gaap_ClassOfStockDomain_019fd476-6736-79c7-b31b-6885daa1407f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019fd476-6736-7693-8ca0-6859406b64a3" 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_019fd476-6736-79c7-b31b-6885daa1407f" xlink:to="loc_us-gaap_RestrictedStockMember_019fd476-6736-7693-8ca0-6859406b64a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationByTypeAxis_019fd476-6736-79e6-925d-89794c6f223a" 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_019fc4c8-e22b-7da6-97b8-25553fe1facd" xlink:to="loc_us-gaap_ContingentConsiderationByTypeAxis_019fd476-6736-79e6-925d-89794c6f223a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-79e6-925d-89794c6f223a_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_019fd476-6736-79e6-925d-89794c6f223a" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-79e6-925d-89794c6f223a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-7d22-b5c6-4bce6a807f4d" 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_019fd476-6736-79e6-925d-89794c6f223a" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-7d22-b5c6-4bce6a807f4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PurchaseConsiderationMember_019fd476-6736-7f7d-882b-3e26b9f9b3d2" xlink:href="algt-20260630.xsd#algt_PurchaseConsiderationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-7d22-b5c6-4bce6a807f4d" xlink:to="loc_algt_PurchaseConsiderationMember_019fd476-6736-7f7d-882b-3e26b9f9b3d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SpecialChargesMember_019fd476-6736-7c5e-9d1b-e0b049252711" xlink:href="algt-20260630.xsd#algt_SpecialChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_019fd476-6736-7d22-b5c6-4bce6a807f4d" xlink:to="loc_algt_SpecialChargesMember_019fd476-6736-7c5e-9d1b-e0b049252711" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019fc4c8-e22b-7904-8e97-77cca125682e" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019fc4c8-e22b-7904-8e97-77cca125682e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionSharePrice_019fc4c8-e22b-70c7-b0da-b93097c94e1a" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_BusinessAcquisitionSharePrice_019fc4c8-e22b-70c7-b0da-b93097c94e1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare_019fc4c8-e22b-70af-891e-4d9f36bd6753" xlink:href="algt-20260630.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableCashPaidPerAcquireeShare_019fc4c8-e22b-70af-891e-4d9f36bd6753" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare_019fc4c8-e22b-7f84-aec5-c0b4cae7b694" xlink:href="algt-20260630.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedandIssuableEntitySharesIssuedPerAcquireeShare_019fc4c8-e22b-7f84-aec5-c0b4cae7b694" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare_019fc4c8-e22b-7fe8-bf7e-9bb8ebc017f1" xlink:href="algt-20260630.xsd#algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableEntitySharesIssuedPerAcquireePerShare_019fc4c8-e22b-7fe8-bf7e-9bb8ebc017f1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019fc4dd-8e48-7b3d-b684-8c877f1e925c" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019fc4dd-8e48-7b3d-b684-8c877f1e925c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfCommonStockSharesIssued_019fd476-47b3-7a8c-8319-7e80501d5011" xlink:href="algt-20260630.xsd#algt_BusinessCombinationNumberOfCommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationNumberOfCommonStockSharesIssued_019fd476-47b3-7a8c-8319-7e80501d5011" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_019fd476-6736-7267-8686-3d5cc33de575" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_019fd476-6736-7267-8686-3d5cc33de575" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationUnrecognizedCompensationCost_019fd476-6736-7d18-a915-7c289932f7ac" xlink:href="algt-20260630.xsd#algt_BusinessCombinationUnrecognizedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationUnrecognizedCompensationCost_019fd476-6736-7d18-a915-7c289932f7ac" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod_019fd476-6736-778f-bc72-81637fe82dfa" xlink:href="algt-20260630.xsd#algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationSalariesAndWagesWeightedAverageTermPeriod_019fd476-6736-778f-bc72-81637fe82dfa" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationConversionOfStockSharesIssued_019fd476-6736-7fb8-b942-9cd2fefb11a6" xlink:href="algt-20260630.xsd#algt_BusinessCombinationConversionOfStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_BusinessCombinationConversionOfStockSharesIssued_019fd476-6736-7fb8-b942-9cd2fefb11a6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019fd476-6736-7a55-a7f2-47184116b197" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019fd476-6736-7a55-a7f2-47184116b197" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019fd476-7ca4-7faf-bed0-fc24eb682efc" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019fd476-7ca4-7faf-bed0-fc24eb682efc" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_GoodwillAndIntangibleAssets_019fd48d-350b-7101-894c-da4e29ed1c9c" xlink:href="algt-20260630.xsd#algt_GoodwillAndIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_algt_GoodwillAndIntangibleAssets_019fd48d-350b-7101-894c-da4e29ed1c9c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019fd4a8-dbb8-74b3-9e58-fa6ad326d72c" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_Goodwill_019fd4a8-dbb8-74b3-9e58-fa6ad326d72c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019fd84c-d307-7a87-981a-a65ab97b3985" 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_019fc4c8-e22b-7dcc-a484-659f62dfb7b7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019fd84c-d307-7a87-981a-a65ab97b3985" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationContingentConsiderationDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationContingentConsiderationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationContingentConsiderationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fc4c8-6646-7c42-b05f-0327b90e2418" 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_019fc4c8-6646-73b4-8083-17a615cd2392" 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_019fc4c8-6646-7c42-b05f-0327b90e2418" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fc4c8-6646-73b4-8083-17a615cd2392" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fc4c8-6646-73b1-b02e-af8df6ebca7f" 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_019fc4c8-6646-73b4-8083-17a615cd2392" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fc4c8-6646-73b1-b02e-af8df6ebca7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-6646-73b1-b02e-af8df6ebca7f_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_019fc4c8-6646-73b1-b02e-af8df6ebca7f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-6646-73b1-b02e-af8df6ebca7f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-6646-7f6c-8fd4-7c2e0372fd4f" 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_019fc4c8-6646-73b1-b02e-af8df6ebca7f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-6646-7f6c-8fd4-7c2e0372fd4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fc4c9-ba8a-7446-9aff-353a07a201ba" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fc4c8-6646-7f6c-8fd4-7c2e0372fd4f" xlink:to="loc_algt_SunCountryMember_019fc4c9-ba8a-7446-9aff-353a07a201ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019fc4c8-6646-7b11-b5c8-bd493c46cf8d" 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_019fc4c8-6646-73b4-8083-17a615cd2392" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019fc4c8-6646-7b11-b5c8-bd493c46cf8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7b11-b5c8-bd493c46cf8d_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_019fc4c8-6646-7b11-b5c8-bd493c46cf8d" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7b11-b5c8-bd493c46cf8d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7809-a050-cec22b96c3a2" 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_019fc4c8-6646-7b11-b5c8-bd493c46cf8d" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7809-a050-cec22b96c3a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019fc4c8-6646-71a7-a937-425a32a42b47" 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_019fc4c8-6646-7809-a050-cec22b96c3a2" xlink:to="loc_us-gaap_CommonStockMember_019fc4c8-6646-71a7-a937-425a32a42b47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember_019fc4c8-6646-705a-a854-4c7761ea36ae" xlink:href="algt-20260630.xsd#algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7809-a050-cec22b96c3a2" xlink:to="loc_algt_PreCombinationValueOfReplacedAndAcceleratedEquityAwardsMember_019fc4c8-6646-705a-a854-4c7761ea36ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_ConsiderationPaidWithRespectToAmazonMember_019fc4c8-6646-7578-8a46-18ca32f8ff5d" xlink:href="algt-20260630.xsd#algt_ConsiderationPaidWithRespectToAmazonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7809-a050-cec22b96c3a2" xlink:to="loc_algt_ConsiderationPaidWithRespectToAmazonMember_019fc4c8-6646-7578-8a46-18ca32f8ff5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_TerminationPaymentUnderTaxReceivableAgreementMember_019fc4c8-6646-7e4b-b36d-600b47a68fcd" xlink:href="algt-20260630.xsd#algt_TerminationPaymentUnderTaxReceivableAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019fc4c8-6646-7809-a050-cec22b96c3a2" xlink:to="loc_algt_TerminationPaymentUnderTaxReceivableAgreementMember_019fc4c8-6646-7e4b-b36d-600b47a68fcd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019fd46b-b607-7aa8-8cc6-cc3941dc0154" 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_019fc4c8-6646-73b4-8083-17a615cd2392" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019fd46b-b607-7aa8-8cc6-cc3941dc0154" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fd46b-b607-7aa8-8cc6-cc3941dc0154_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_019fd46b-b607-7aa8-8cc6-cc3941dc0154" xlink:to="loc_us-gaap_ClassOfStockDomain_019fd46b-b607-7aa8-8cc6-cc3941dc0154_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019fd46b-b607-7a84-beac-8ccd6b8cdb03" 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_019fd46b-b607-7aa8-8cc6-cc3941dc0154" xlink:to="loc_us-gaap_ClassOfStockDomain_019fd46b-b607-7a84-beac-8ccd6b8cdb03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019fd46b-f3bf-7827-814d-2bf9a2efa90f" 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_019fd46b-b607-7a84-beac-8ccd6b8cdb03" xlink:to="loc_us-gaap_RestrictedStockMember_019fd46b-f3bf-7827-814d-2bf9a2efa90f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019fc4c8-6646-7547-81fb-69007be3bb83" 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_019fc4c8-6646-7c42-b05f-0327b90e2418" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019fc4c8-6646-7547-81fb-69007be3bb83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_019fc4c8-6646-7ec2-9260-85a93c3c1adb" 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_019fc4c8-6646-7c42-b05f-0327b90e2418" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_019fc4c8-6646-7ec2-9260-85a93c3c1adb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019fc4c8-6646-7d43-8c88-b1dd519a6b92" 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_019fc4c8-6646-7c42-b05f-0327b90e2418" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019fc4c8-6646-7d43-8c88-b1dd519a6b92" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationPreliminaryAllocationDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationPreliminaryAllocationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationPreliminaryAllocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" 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_019fd44c-244c-71bf-9f91-5988a07c82cf" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fd44c-244c-71bf-9f91-5988a07c82cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fd44c-244c-740f-96af-6494ebc613ca" 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_019fd44c-244c-71bf-9f91-5988a07c82cf" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fd44c-244c-740f-96af-6494ebc613ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd44c-244c-740f-96af-6494ebc613ca_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_019fd44c-244c-740f-96af-6494ebc613ca" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd44c-244c-740f-96af-6494ebc613ca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd44c-244c-71f5-ad5d-7178c2ad7ea2" 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_019fd44c-244c-740f-96af-6494ebc613ca" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd44c-244c-71f5-ad5d-7178c2ad7ea2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fd44c-244c-739c-b748-86050f5fd791" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd44c-244c-71f5-ad5d-7178c2ad7ea2" xlink:to="loc_algt_SunCountryMember_019fd44c-244c-739c-b748-86050f5fd791" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fd44f-d7a2-751f-a193-dd22d603b2ae" 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_019fd44c-244c-71bf-9f91-5988a07c82cf" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fd44f-d7a2-751f-a193-dd22d603b2ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd44f-d7a2-751f-a193-dd22d603b2ae_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_019fd44f-d7a2-751f-a193-dd22d603b2ae" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd44f-d7a2-751f-a193-dd22d603b2ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd44f-d7a2-77fb-89e5-36a0d081f8b1" 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_019fd44f-d7a2-751f-a193-dd22d603b2ae" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd44f-d7a2-77fb-89e5-36a0d081f8b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_019fd44f-d7a2-7a26-90b5-b3bd50aad76a" 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_019fd44f-d7a2-77fb-89e5-36a0d081f8b1" xlink:to="loc_us-gaap_TrademarksMember_019fd44f-d7a2-7a26-90b5-b3bd50aad76a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019fd450-1452-70c6-8825-8b6008c09229" 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_019fd44f-d7a2-77fb-89e5-36a0d081f8b1" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019fd450-1452-70c6-8825-8b6008c09229" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_019fd44c-244c-7fab-a513-fc1c8dc1cada" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_019fd44c-244c-7fab-a513-fc1c8dc1cada" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash_019fd44c-244c-7d3a-9775-ea25c555825c" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredRestrictedCash_019fd44c-244c-7d3a-9775-ea25c555825c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment_019fd44c-244c-78d7-b420-492d9198c56f" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermInvestment_019fd44c-244c-78d7-b420-492d9198c56f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019fd44c-244c-7bd5-bac8-57b3dfffd926" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019fd44c-244c-7bd5-bac8-57b3dfffd926" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits_019fd44c-244c-7f08-88d8-04267edaa81e" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredShortTermLessorMaintenanceDeposits_019fd44c-244c-7f08-88d8-04267edaa81e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet_019fd44c-244c-79de-9ef1-f0d363a617a7" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredExpendablePartsSuppliesAndFuelNet_019fd44c-244c-79de-9ef1-f0d363a617a7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets_019fd44c-244c-76c9-934e-2376339aeefd" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredPrepaidExpensesAndOtherCurrentAssets_019fd44c-244c-76c9-934e-2376339aeefd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019fd44c-244c-72e0-8345-1ad64b2daf17" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019fd44c-244c-72e0-8345-1ad64b2daf17" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019fd44c-244c-7518-a9ca-a2b3b90b46c9" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_Goodwill_019fd44c-244c-7518-a9ca-a2b3b90b46c9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019fd4aa-4fd1-737c-9218-07e6a18e87a9" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019fd4aa-4fd1-737c-9218-07e6a18e87a9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment_019fd44c-244c-7752-92f4-2200235b1141" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredLongTermInvestment_019fd44c-244c-7752-92f4-2200235b1141" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet_019fd44c-244c-7516-80c9-7e9d7cc5d0a8" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredOperatingLeaseRightOfUseAssetsNet_019fd44c-244c-7516-80c9-7e9d7cc5d0a8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets_019fd44c-244c-787e-9757-12f617f13a85" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedAssetAcquiredDepositsAndOtherAssets_019fd44c-244c-787e-9757-12f617f13a85" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_019fd44c-244c-7162-8f5e-38536f7831b0" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_019fd44c-244c-7162-8f5e-38536f7831b0" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_019fd44c-244c-7147-96e4-bc47b20aff1a" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_019fd44c-244c-7147-96e4-bc47b20aff1a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_019fd44c-244c-754c-8b1c-6ab92ab795b4" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_019fd44c-244c-754c-8b1c-6ab92ab795b4" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent_019fd44c-244c-7e86-bfe8-3e4d7f249d7c" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesCurrent_019fd44c-244c-7e86-bfe8-3e4d7f249d7c" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability_019fd44c-244c-7573-ac00-d28a2d1fa0b4" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedAirTrafficLiability_019fd44c-244c-7573-ac00-d28a2d1fa0b4" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability_019fd44c-244c-71a0-9c9e-37a0fbd8ed6c" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentLoyaltyProgramLiability_019fd44c-244c-71a0-9c9e-37a0fbd8ed6c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts_019fd44c-244c-7c59-8fa0-ed1b3e6e9753" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedCurrentMaturitiesOfLongTermDebtAndFinanceLeaseObligationsNetOfRelatedCosts_019fd44c-244c-7c59-8fa0-ed1b3e6e9753" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts_019fd44c-244c-7d9d-9442-4d3842f71d16" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLongTermDebtAndFinanceLeaseObligationsNetOfCurrentMaturitiesAndRelatedCosts_019fd44c-244c-7d9d-9442-4d3842f71d16" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019fd44c-244c-780a-925f-3f4a3df97c64" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019fd44c-244c-780a-925f-3f4a3df97c64" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent_019fd44c-244c-73ba-97f0-1c4fed2f4d1e" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedOperatingLeaseLiabilitiesNoncurrent_019fd44c-244c-73ba-97f0-1c4fed2f4d1e" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent_019fd44c-244c-7e6e-a9f6-784a6b553fb6" xlink:href="algt-20260630.xsd#algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_algt_BusinessCombinationRecognizedLiabilityAssumedLoyaltyProgramLiabilityNoncurrent_019fd44c-244c-7e6e-a9f6-784a6b553fb6" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_019fd44c-244c-717d-bf4c-aa7a52fa7bbf" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_019fd44c-244c-717d-bf4c-aa7a52fa7bbf" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019fd44c-244c-73da-b3af-c6aa6f1b23ef" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019fd44c-244c-73da-b3af-c6aa6f1b23ef" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_019fd44c-244c-7664-9a9d-5107e480f7a0" 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_019fd44c-244c-7169-a3e4-2ffd947d6ddb" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_019fd44c-244c-7664-9a9d-5107e480f7a0" xlink:type="arc" order="26"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationIntangibleAssetAcquiredIndefiniteLivedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fd450-75be-7158-b235-71d02043d40d" 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_019fd450-75be-755b-9a74-b4102bed8e22" 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_019fd450-75be-7158-b235-71d02043d40d" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fd450-75be-755b-9a74-b4102bed8e22" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fd450-75be-77bb-b674-2fb7af232d72" 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_019fd450-75be-755b-9a74-b4102bed8e22" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fd450-75be-77bb-b674-2fb7af232d72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd450-75be-77bb-b674-2fb7af232d72_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_019fd450-75be-77bb-b674-2fb7af232d72" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd450-75be-77bb-b674-2fb7af232d72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd450-75be-701c-bb64-62b6df6d1ae6" 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_019fd450-75be-77bb-b674-2fb7af232d72" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd450-75be-701c-bb64-62b6df6d1ae6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fd450-75be-743f-9ed3-c9de7af4e54d" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd450-75be-701c-bb64-62b6df6d1ae6" xlink:to="loc_algt_SunCountryMember_019fd450-75be-743f-9ed3-c9de7af4e54d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fd450-75be-73d5-8f58-6e1e21661606" 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_019fd450-75be-755b-9a74-b4102bed8e22" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019fd450-75be-73d5-8f58-6e1e21661606" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd450-75be-73d5-8f58-6e1e21661606_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_019fd450-75be-73d5-8f58-6e1e21661606" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd450-75be-73d5-8f58-6e1e21661606_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd450-75be-732f-9aba-a2ad36e487d6" 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_019fd450-75be-73d5-8f58-6e1e21661606" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019fd450-75be-732f-9aba-a2ad36e487d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_019fd450-75be-792c-9eac-e739fbe39295" 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_019fd450-75be-732f-9aba-a2ad36e487d6" xlink:to="loc_us-gaap_TrademarksMember_019fd450-75be-792c-9eac-e739fbe39295" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019fd450-75be-71fe-98c1-8cc9d5a44c1b" 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_019fd450-75be-732f-9aba-a2ad36e487d6" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019fd450-75be-71fe-98c1-8cc9d5a44c1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019fd450-75be-7377-a0b3-c34d80c46666" 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_019fd450-75be-7158-b235-71d02043d40d" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019fd450-75be-7377-a0b3-c34d80c46666" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019fd450-75be-71e9-b9ef-87f2a8584acb" 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_019fd450-75be-7158-b235-71d02043d40d" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019fd450-75be-71e9-b9ef-87f2a8584acb" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationEmployeeBenefitPlansActivityDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationEmployeeBenefitPlansActivityDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationEmployeeBenefitPlansActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" 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_019fd459-0506-7fbe-abfb-6ababcdd3a3c" 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_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fd459-0506-7fbe-abfb-6ababcdd3a3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fd459-0506-7ba0-9c09-586043b635e2" 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_019fd459-0506-7fbe-abfb-6ababcdd3a3c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fd459-0506-7ba0-9c09-586043b635e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd459-0506-7ba0-9c09-586043b635e2_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_019fd459-0506-7ba0-9c09-586043b635e2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd459-0506-7ba0-9c09-586043b635e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd459-0506-775b-b52a-c7c6e87b143f" 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_019fd459-0506-7ba0-9c09-586043b635e2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd459-0506-775b-b52a-c7c6e87b143f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fd459-0506-754f-9315-2dceb68cd1b8" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd459-0506-775b-b52a-c7c6e87b143f" xlink:to="loc_algt_SunCountryMember_019fd459-0506-754f-9315-2dceb68cd1b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfStockSharesGranted_019fd459-0506-70b8-b667-11bb2f9658ac" xlink:href="algt-20260630.xsd#algt_BusinessCombinationNumberOfStockSharesGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_algt_BusinessCombinationNumberOfStockSharesGranted_019fd459-0506-70b8-b667-11bb2f9658ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfStockSharesVested_019fd459-0506-7fdf-afde-41ba8d8f5c91" xlink:href="algt-20260630.xsd#algt_BusinessCombinationNumberOfStockSharesVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_algt_BusinessCombinationNumberOfStockSharesVested_019fd459-0506-7fdf-afde-41ba8d8f5c91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfStockSharesForfeited_019fd459-0506-73a3-8960-06a99bc838c9" xlink:href="algt-20260630.xsd#algt_BusinessCombinationNumberOfStockSharesForfeited"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_algt_BusinessCombinationNumberOfStockSharesForfeited_019fd459-0506-73a3-8960-06a99bc838c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationNumberOfStockSharesNonVested_019fd459-0506-7d80-b355-f3d7f5bfaf26" xlink:href="algt-20260630.xsd#algt_BusinessCombinationNumberOfStockSharesNonVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_algt_BusinessCombinationNumberOfStockSharesNonVested_019fd459-0506-7d80-b355-f3d7f5bfaf26" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_BusinessCombinationGrantDateFairValue_019fd459-2f54-70bc-908f-8de6f08c0df0" xlink:href="algt-20260630.xsd#algt_BusinessCombinationGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019fd459-0506-7929-921a-a13000db5803" xlink:to="loc_algt_BusinessCombinationGrantDateFairValue_019fd459-2f54-70bc-908f-8de6f08c0df0" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationProFormaInformationDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationProFormaInformationDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/AcquisitionofSunCountryAirlinesHoldingsIncBusinessCombinationProFormaInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019fd475-791a-7833-a053-f872434ce746" 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_019fd475-791a-7b35-8ed5-71c88327a1b6" 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_019fd475-791a-7833-a053-f872434ce746" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019fd475-791a-7b35-8ed5-71c88327a1b6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019fd475-791a-722f-8385-7c87b0319ff2" 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_019fd475-791a-7b35-8ed5-71c88327a1b6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019fd475-791a-722f-8385-7c87b0319ff2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd475-791a-722f-8385-7c87b0319ff2_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_019fd475-791a-722f-8385-7c87b0319ff2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd475-791a-722f-8385-7c87b0319ff2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd475-791a-79b6-8870-6c5c20dcc292" 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_019fd475-791a-722f-8385-7c87b0319ff2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd475-791a-79b6-8870-6c5c20dcc292" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_SunCountryMember_019fd475-791a-709d-a062-3f28c45ff0b4" xlink:href="algt-20260630.xsd#algt_SunCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019fd475-791a-79b6-8870-6c5c20dcc292" xlink:to="loc_algt_SunCountryMember_019fd475-791a-709d-a062-3f28c45ff0b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_019fd475-791a-7892-a3ca-3c54cce50798" 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_019fd475-791a-7833-a053-f872434ce746" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_019fd475-791a-7892-a3ca-3c54cce50798" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_019fd475-791a-799b-9ff6-dad12142e431" 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_019fd475-791a-7833-a053-f872434ce746" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_019fd475-791a-799b-9ff6-dad12142e431" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www2.allegiantair.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="algt-20260630.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www2.allegiantair.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_us-gaap_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019fc4bf-aa60-7981-a6ad-e2e51bd136da" 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_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:to="loc_us-gaap_CreditFacilityAxis_019fc4bf-aa60-7981-a6ad-e2e51bd136da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc4bf-aa60-7981-a6ad-e2e51bd136da_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_019fc4bf-aa60-7981-a6ad-e2e51bd136da" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc4bf-aa60-7981-a6ad-e2e51bd136da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fc4bf-aa60-73d8-835d-846debbb566a" 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_019fc4bf-aa60-7981-a6ad-e2e51bd136da" xlink:to="loc_us-gaap_CreditFacilityDomain_019fc4bf-aa60-73d8-835d-846debbb566a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019fc4bf-aa60-700c-80d6-e5c5105c2783" 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_019fc4bf-aa60-73d8-835d-846debbb566a" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019fc4bf-aa60-700c-80d6-e5c5105c2783" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fc4bf-aa60-7d32-8d87-8032cfbab04d" 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_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fc4bf-aa60-7d32-8d87-8032cfbab04d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7d32-8d87-8032cfbab04d_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_019fc4bf-aa60-7d32-8d87-8032cfbab04d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7d32-8d87-8032cfbab04d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" 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_019fc4bf-aa60-7d32-8d87-8032cfbab04d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementDBJ22Member_019fc4bf-aa60-76b6-8000-9759a0355846" xlink:href="algt-20260630.xsd#algt_CreditAgreementDBJ22Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" xlink:to="loc_algt_CreditAgreementDBJ22Member_019fc4bf-aa60-76b6-8000-9759a0355846" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementPDP3Member_019fc4bf-aa60-79b8-9f41-4986375157f5" xlink:href="algt-20260630.xsd#algt_CreditAgreementPDP3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" xlink:to="loc_algt_CreditAgreementPDP3Member_019fc4bf-aa60-79b8-9f41-4986375157f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementMeritMember_019fc4bf-aa60-7ac1-96ed-2f8ff71f1d34" xlink:href="algt-20260630.xsd#algt_CreditAgreementMeritMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" xlink:to="loc_algt_CreditAgreementMeritMember_019fc4bf-aa60-7ac1-96ed-2f8ff71f1d34" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_CreditAgreementCANSB234Member_019fc4bf-aa60-713b-9005-24e20379d6be" xlink:href="algt-20260630.xsd#algt_CreditAgreementCANSB234Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fc4bf-aa60-7790-aae2-64a22e756f99" xlink:to="loc_algt_CreditAgreementCANSB234Member_019fc4bf-aa60-713b-9005-24e20379d6be" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fc4bf-aa60-728b-98e7-67ab716388ef" 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_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fc4bf-aa60-728b-98e7-67ab716388ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc4bf-aa60-728b-98e7-67ab716388ef_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_019fc4bf-aa60-728b-98e7-67ab716388ef" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc4bf-aa60-728b-98e7-67ab716388ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fc4bf-aa60-7492-a943-2e73c868abf2" 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_019fc4bf-aa60-728b-98e7-67ab716388ef" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fc4bf-aa60-7492-a943-2e73c868abf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019fc4bf-aa60-70a2-83da-c14b93c73831" 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_019fc4bf-aa60-7492-a943-2e73c868abf2" xlink:to="loc_us-gaap_LineOfCreditMember_019fc4bf-aa60-70a2-83da-c14b93c73831" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019fc4bf-aa60-7a41-a079-087b39d2c50e" 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_019fc4bf-aa60-7492-a943-2e73c868abf2" xlink:to="loc_us-gaap_SecuredDebtMember_019fc4bf-aa60-7a41-a079-087b39d2c50e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fc4bf-aa60-7360-8f5e-df21356aaf9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019e42a3-b276-7cd2-b1bd-7353fb5b5239" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fc4bf-aa60-7360-8f5e-df21356aaf9a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fc4bf-aa60-7360-8f5e-df21356aaf9a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fc4bf-aa60-7360-8f5e-df21356aaf9a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fc4bf-aa60-7360-8f5e-df21356aaf9a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fc4bf-aa60-7c4a-8829-09f478f3eb6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fc4bf-aa60-7360-8f5e-df21356aaf9a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fc4bf-aa60-7c4a-8829-09f478f3eb6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fc4bf-aa60-704a-a5f7-9d084fb5abc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fc4bf-aa60-7c4a-8829-09f478f3eb6f" xlink:to="loc_us-gaap_SubsequentEventMember_019fc4bf-aa60-704a-a5f7-9d084fb5abc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019fc4bf-aa60-7b4a-867c-e77422325e94" 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_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_us-gaap_LineOfCredit_019fc4bf-aa60-7b4a-867c-e77422325e94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc4bf-aa60-7e4a-b2f8-36bba6d2f9b2" 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_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019fc4bf-aa60-7e4a-b2f8-36bba6d2f9b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019fc4bf-aa60-70fc-a845-9c18464f52b2" 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_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_us-gaap_DebtInstrumentTerm_019fc4bf-aa60-70fc-a845-9c18464f52b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_019fc4bf-aa60-7413-9fd8-fe6693bbbb5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_us-gaap_RepaymentsOfDebt_019fc4bf-aa60-7413-9fd8-fe6693bbbb5f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_algt_NumberOfBoeingAircrafts_019fc4bf-aa60-75a8-93f0-5adf70c763d6" xlink:href="algt-20260630.xsd#algt_NumberOfBoeingAircrafts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fc4bf-aa60-75e3-a329-6435916dedb3" xlink:to="loc_algt_NumberOfBoeingAircrafts_019fc4bf-aa60-75a8-93f0-5adf70c763d6" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
