Blunder

A blunder is a move with severe consequences. It may lose a valuable piece, allow checkmate, throw away a win, or turn a defensible position into a loss. The move is still legal; the term describes its effect, not a violation of the rules.

Severity is determined by what the opponent can achieve with a correct reply, even if that reply is missed during the game. A move therefore remains a blunder when the opponent fails to punish it. A long-term decision can also qualify without losing a piece or pawn immediately if the evaluation, the judgment of which side stands better, deteriorates decisively.

Automated analysis systems distinguish an inaccuracy, a mistake, and a blunder, but their thresholds vary. In human analysis, the label also depends on context: the size of the damage, how easily the opponent can punish it, and the previous state of the game all matter. In game analysis, reconstruct both the missed threat and the reason it was overlooked.