Is there any definition of "mistake" or "blunder"

First of all "blunder" does not necessarily mean, that the move is obviously wrong. There are obvious "inaccuracies" and tricky "blunders". Also a better engine eval does not necessarily mean, that the move is "better" in a practical game: the "optimal" line might be more difficult for a human (in the way, that you have to find some tricky non-intuitive moves)
@drwerewolf said in #1:
and a few moves later when I went from +14.6 down to +6.3. Those feel pretty BLUNDERific to me



The position is interesting. I certainly would not have played your "+6.3 move" Nxd5, because after a few seconds I still don't see a trivial win after Bxd5 Qxd5 Qxe3+ and Qxd3. The machine recommendation h4 looks ofc good. Personally I would have played quickly the greedy and simple hxg4 instead and the machine likes that move too (+12 eval).

@drwerewolf
Yes.
See: en.m.wikipedia.org/wiki/Chess_annotation_symbols#Evaluation_symbols

I did win the game ... oberschlumpf's reply above captures the game in a way I didn't know was possible (newbie to these forums).

Thanks for the feedback.

Also, note that engine evaluation is way different than human analysis. At this stage you don't need to delve into depth but you should know that Stockfish (the engine) evaluates position based on relative moves played. Engine doesn't distinguish between blunder and okayish but still winning kind of moves.
So, change in evaluation from +14 to +6 doesn't matter if you are already having an advanced position of the game and winning seems bit away.