To attack (a chess opponent's king) in such a manner that no escape or defense is possible, thus ending the game.
To defeat completely.
n.
A move that constitutes an inescapable and indefensible attack on a chess opponent's king.
The position or condition of a king so attacked.
Utter defeat.
interj. Used to declare the checkmate of an opponent's king in chess.
[Middle English chekmat, from Old French eschec mat, from Arabic šāh māt, the king is dead : šāh, king (from Persian shāh; see shah) + māt, died (from earlier māta, to die; see mwt in Semitic roots).]
[chek-meyt] 
noun, verb, -mat⋅ed, -mat⋅ing, interjection 