Checkmate (often shortened to mate) is a game position in chess (and in other board games of the chaturanga family) in which a player's king is threatened with capture (i.e. is in check) and there is no way to counter the threat. Or, simply put, the king is under direct attack and cannot avoid being captured.

Delivering checkmate is the ultimate goal in chess: a player who is checkmated loses the game. In chess the king is never actually captured – the game ends as soon as the king is checkmated because checkmate leaves the defending player with no legal moves.^{[note 1]} In practice, most players resign an inevitably lost game before being checkmated, and it is considered bad etiquette to continue playing in a completely hopeless position (Burgess 2000:481).^{[note 2]}

If a player's king is in check but the threat can be met, then it is not in checkmate. If a player is not in check but has no legal move (that is, every possible move would place the king in check), then it is stalemate, and the game immediately ends in a draw. (See rules of chess.)

A checkmating move is recorded in algebraic notation using the hash symbol (#) – for example, 34.Qh8#. (The symbol "++" is sometimes if rarely used, which can also mean double check.)