losing move
In commentaries of professional games, the term losing move is used for the move at which the eventual loser still had his chances, but chose a clearly inferior play. The winner next made no mistakes so that even if the loser didn't make any either, he still could not recover the loss.
Sometimes magazine commentaries, for example in the now-defunct Kido, concentrate on the losing move - the point of no return in the game. This is however not so useful for amateur study, since it mostly says 'disregard anything that happens next'.
IMX, trying to find the losing move in my own games has been quite useful. It is seldom an easy exercise.
emeraldemon: I try to look for the losing move in my own games also, and I think it has helped a lot to focus my study efforts. I also think it's valuable for the teacher: /Example