I've never quite understood what defines the end of the middlegame and now the ENDGAME. I've seen so called endgames (as coined by others) with all the major pieces still on the board, Queen and both rooks for each side with 5 or six pawns apiece. Huh???

Is there a clear definition?

Not really. The term is rather variable, and there isn't really a hard-and-fast rule, though people will try to give you some (differing ones) anyway.