That's an American definition of "middle class", though, and the problem with it is that's rather fuzzy, since it merely delineates some arbitrary income range. In the UK and other places, the term was linked more to being your boss (e.g. being an independent professional and/or owning a small business). Of course, now that (as a song in my country put it), "one has to study to be a slave", this may be harder to cleanly distinguish.