Wikipedia defines Usability as:
A qualitative attribute that assesses how easy user interfaces are to use. The word “usability” also refers to methods for improving ease-of-use during the design process..
Whether something is easy to use depends on what it is being used for, which is partly a function of the intentions of the designer, and partly of the expectations of the user.
Usability can therefore only be achieved by a process which incorporates feedback from actual testing with typical real users. Often more than one iteration may be required, with improvements being incorporated at each step.
The design of the actual test may involve measuring performance at actual tester-defined tasks, or may involve making observations of unguided user behaviour and purposes - or indeed a mixture of the two. By definition since a particular website has a particular purpose, and attracts a particular clientele with particular needs, a usability test schedule must be individually tailored.
Usability is not the same as Accessibility, which in relation to UK legal obligations is concerned with the degree to which a minority group may be disadvantaged in its user experience. Poor Accessibility would clearly involve reduced Usability for the minority, and both aspects will need serious attention to satisfy the law. Poor Usability could in principle be coupled with equal Accessibility to all - but it would be a strange organisation which set out to provide something equally awful for everyone.
The testing and design effort involved in meeting Accessibility Guidelines (which are not simply technical) will in practice probably be beneficial in increasing the Usability for all users.
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