Just very briefly for 'defintive':
a) not able to be changed or improved
b) considered to be the best of its type
c) firm, final, and complete; not to be questioned or changed
(from the Cambridge Dictionairy.
I think this is a place to start, but Cambridge likely needs to think this one out a bit more. Anything can be changed but perhaps not improved. Our adaptions are obviously changes, as are any parody and heavy references to works of art. I suppose the definition could mean no changes that would result in improvement as measured by popularity, familiarity, etc.
Also, I think I have been confusing the subjective "best" vs. more objective "best" as the term has been used throughout the thread. I may personally think a performance is the best but that doesn't make it the objective best. "Definitive" doesn't mean a subjective best but the more objective best as again established by popularity, familiarity, etc.
I suspect that the editors, CCs and heavy users of SHS will often personally prefer a performance other than the definitive recording, but they should also be objective enough to recognize that personal preference doesn't equate to definitive.