The definition you provided is pretty much the standard one. Wording may vary, emphasis may vary, and the definition leaves out a lot of details. And therein lies the rub: the devil is in the details. The deeper you go, the more detail is revealed, and that is where discussion, argument and disagreement arise - among scientists, at any rate.
Why is there even a debate over what evolution is and is not?
There isn't. Not among biologists (and ALL of the life sciences disciplines). Only those who, for whatever reason, dislike the non-scientific
implications - or at least what they see as the implications - argue over what it is and is not. Most of the threads on this board are geared to correcting - sometimes with a hammer - misconceptions by non-biologists. And in-house over the details.