losing move

A losing move is an irretrievable mistake in play: it loses the game or gives away a decisive amount of value. The move changes the position so unfavorably that the lost result or advantage cannot be recovered through normal subsequent play.

In a game record or analysis, identifying the losing move helps locate the error that decisively determined the outcome. It is more severe than an ordinary inaccuracy or suboptimal move because it materially decides the game. However, whether a move is truly losing depends on accurate positional evaluation and reading. A move that looks bad may still be playable.