What is usability?

What is usability? I've read definitions that say usability and user-experience are entirely separate; that usability is a subsection of user experience; or that they are just terms that express different facets of the same subject matter.

Although I now see that many aspects of both usability and user experience design are common sense concepts and practices, it's only over the past year or so that I've become consciously aware of the reason these things can exist as a separate discipline from web design, and the level of complexity therein. Myself, I started out 11 years ago as a web developer, became a designer, and now I'm trying to get a better understanding of usability.