The Collaborative International Dictionary of English v.0.48:

Checkmate \Check"mate\ (-m[=a]t), v. t. [imp. & p. p.
   Checkmated; p. pr. & vb. n. Checkmating.]
   1. (Chess) 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.
      [1913 Webster]

   2. To defeat completely; to terminate; to thwart.
      [1913 Webster]

            To checkmate and control my just demands. --Ford.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Checkmate \Check"mate\ (ch[e^]k"m[=a]t), n. [F. ['e]chec et mat,
   fr. Per. sh[=a]h m[=a]t checkmate, lit., the king is dead,
   fr. Ar. m[=a]ta he died, is dead. The king, when made
   prisoner, or checkmated, is assumed to be dead, and the game
   is finished. See Chess.]
   1. The position in the game of chess when a king is in check
      and cannot be released, -- which ends the game.
      [1913 Webster]

   2. A complete check; utter defeat or overthrow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

checkmate
    n 1: complete victory
    2: a chess move constituting an inescapable and indefensible
       attack on the opponent's king [syn: checkmate, mate]
    v 1: place an opponent's king under an attack from which it
         cannot escape and thus ending the game; "Kasparov
         checkmated his opponent after only a few moves" [syn:
         checkmate, mate]