miss - defined by chess.com

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a move that missed a tactical opportunity or a chance to punish the opponent.

Taking the D4 pawn with the E5 pawn is the best move

Taking the D4 pawn as black with the knight is a 'miss'

I think they just defined "blunder" more specifically as a move that gives your opponent chances, and added "miss" to fill in the gap for when you miss a chance given to you. 

In the past, misses would've been called blunders.

Stockfish favors the capture of the D4 pawn with the E5 pawn (-4.78) vs (-1.27) with the knight capture.

I think its also a miss when theres only one good move and you dont find it. for example if theres a move thats +5 and the rest are +2 and you play a move thats +2, that would be a "good" or "inaccuracy" in the past because its still okay just not the best. i believe they have made it so said move would be a miss because you missed a chance to be +5 but youre still doing ok

My opponent 'blundered' on the next move. Taking on D4 with the knight on F3 opened the queen to attack. The miss worked out for me because the opponent blundered and resigned after losing his queen.

There were two changes.

1. Improved Blunder Classification - It used to be that if the engine evaluation went down a lot, then you’d get a blunder. However, this isn’t how humans tend to evaluate. Engine evals can drop due to weird positional considerations that are hard to understand and require long-term precise play from your opponent to punish. In contrast, blunders are typically considered very immediate and concrete. The new blunder definition requires your move to lose material or allow a forced checkmate, in addition to the large drop in engine eval.
2. New Miss Classification - Sometimes a move is bad not because something went horribly wrong with your own position, but because you failed to punish your opponent’s bad move. For example: you’re in an even game, your opponent gives you the opportunity to fork and win a piece, but you just make a normal continuing move as if nothing happened. These cases will be covered by a new classification called Miss, which will be denoted with a Ø glyph.

'thing' is sometimes a term associated with a non canonical perception beyond the scope of the conversation.