Usability is not a quality that exists in any real or absolute sense. Perhaps it can be best summed up as being a general quality of the appropriateness to a purpose of any particular artefact. This notion is neatly summed up by Terry Pratchett in his novel Moving Pictures:

In just the same way, the usability of any tool or system has to be viewed in terms of the context in which it is used, and its appropriateness to that context. With particular reference to information systems, this view of usability is reflected in the current draft international standard ISO 9241-11 and in the European Community ESPRIT project MUSiC (Measuring Usability of Systems in Context) (e.g. Bevan et al., 1991). In general, it is impossible to specify the usability of a system (i.e. its fitness for purpose) without first defining who are the intended users of the system, the tasks those users will perform with it, and the characteristics of the physical, organizational and social environment in which it will be used.