Quality of life is of central concern in evaluative research; improved quality of life is probably the most desirable outcome of all health care policies. However, definitions of quality of life are as numerous and inconsistent as the methods of assessing it. Stemming from a larger piece of work looking at the definition and measurement of quality of life, this paper highlights the lack of a consensus definition of quality of life by means of a taxonomy of definitions that emerge from the literature. The paper describes and gives examples of four main types of definition which make up the taxonomy: global (type I); component (type II); focused (type III); and combination definitions (type IV). In addition, an outline of factors influencing the definition of quality of life is given, and an alternative strategy for both defining and measuring the concept (the use of lay definitions) is suggested.