Comprehensive definitions of social issues and populations can set the stage for the development of responsive policies and practices. Yet despite the rise of late- life homelessness, the phenomenon remains narrowly understood and ill-defined. This paper and the definition that ensued is based on the reconceptualization of interview data derived from a critical ethnography conducted in Montreal, Canada with older homeless persons (N=40) and service providers (N=20). Our analysis suggests that definitions of late life homelessness must include four intersecting components: 1) age, eligibility, and access to services ; 2) disadvantage over the life course and across time; 3) social and spatial processes of exclusion that necessitate aging in 'undesirable' places; and 4) unmet needs that result from policy inaction and non-response. The new definition derived from these structural and relational components captures how the service gaps and complex needs identified in earlier works are shaped by delivery systems and practices whose effect is compounded over time. It provides an empirically grounded and conceptually solid foundation for the development of better responses to address homelessness in late life.