The use of the term 'frailty' has shown tremendous growth in the last 15 years, but this has not been accompanied by a widely accepted definition, let alone agreed-upon measures. In this paper, we review approaches to the definition and measurement of frailty and discuss the relationship between frailty, aging and disability. Two trends are evident in definitions, which often trade off comprehensiveness for precision: frailty can be seen as being synonymous with a single-system problem or as a multisystem problem. The essential feature of frailty is the notion of risk due to instability (itself suggesting a balance of many factors), and has been only poorly measured to date. Future models of frailty should incorporate more precise operationalisation of the probability of frailty and better explain the relationship between disease, disability and frailty.