  According to Brooke [1] "Usability does not exist in any absolute sense; it
can only be defined with reference to particular contexts." That is, one cannot
speak of usability without specifying what that particular usability is
characterized by. Driven by the feedback of a reviewer at an international
conference, I explore in which way one can precisely specify the kind of
usability they are investigating in a given setting. Finally, I come up with a
formalism that defines usability as a quintuple comprising the elements level
of usability metrics, product, users, goals and context of use. Providing
concrete values for these elements then constitutes the investigated type of
usability. The use of this formalism is demonstrated in two case studies.
  [1] J. Brooke. SUS: A "quick and dirty" usability scale. In P. W. Jordan, B.
Thomas, B. A. Weerdmeester, and A. L. McClelland, editors, Usability Evaluation
in Industry. Taylor and Francis, 1996.
