According to Brooke* "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. * 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.