There is as yet no widely-accepted definition of pre-frailty. We aimed to identify and examine definitions of pre-frailty in the literature to characterise important features and factors contributing to the construct using a systematic review approach with a qualitative analysis. PubMed, PsycINFO, Embase, Cochrane Library, ASSIA, and CINAHL databases were searched for studies conducted in any settings providing a definition or description of pre-frailty, published in English, between January 2000 and July 2018. Seventy-seven studies met the inclusion criteria. No consensus definition of pre-frailty was evident in the literature. Four main themes were identified using thematic analysis: (1) Pre-frailty as a prodromal, multi-factorial concept; (2) Physical, social, cognitive and nutritional subtypes; (3) Operational definitions; and (4) Outcomes. We propose a comprehensive definition suggesting that pre-frailty is a multi-dimensional concept, an early and reversible risk-state before frailty that can lead to negative healthcare outcomes, which is defined operationally by existing frailty screening and assessment tools.