According to Wikipedia:
Usability is a term used to denote the ease with which people can employ a particular tool or other human-made object in order to achieve a particular goal.
And further on, it’s stated:
Although most people focus on interface design when they hear this term, the definition allows room for more.
For example, I’m now toying with the idea of replacing “Accessible” with “Usable” in the context of the recently blogged interest in Accessible Parallel Computing.