The position in the game of chess when a king is in check and cannot be released, -- which ends the game.(noun)
source: webster1913
verb (v)
place an opponent's king under an attack from which it cannot escape and thus ending the game(verb.competition)
Example: Kasparov checkmated his opponent after only a few moves.
source: wordnet30
To check (an adversary's king) in such a manner that escape in impossible; to defeat (an adversary) by putting his king in check from which there is no escape.(verb)