Checkmate (frequently shortened to mate) is a situation in chess in which one player cannot avoid their king being captured on the next move - it is a check from which there is no escape. A player who is checkmated loses the game. Delivering checkmate is the ultimate goal in chess (although not all games end in checkmate - often a player resigns before checkmate is administered, or the game may end in a draw in several ways).
It should be noted that the actual capture of the king is not played - the game ends as soon as a position arises in which the capture is unavoidable.
The fastest a player can cause a checkmate is 2 moves. This occurs in fool's mate when a player moves their kings bishop's pawn one or two squares and kings knight's pawn two squares, and their opponent moves their queen to the file of the opponent's king's rook (1. f3 e5 2. g4 Qh4# in algebraic notation).
If a player's king is threatened with capture, but that threat can be met, then the king is said to be in check (a check may be met by moving the king to a safe square, by interposing a piece between the king and the threatening piece (assuming the attacking piece is not directly next to the king and is also not a knight), or by capturing the threatening piece). If a player is not in check but has no legal move (that is, no move which does not allow their king to be captured), the result of the game is stalemate.
The term is an alteration or Hobson-Jobson of the Persian phrase "Shah Mat" which means, literally, "the King is dead."
Some common or notable mating patterns have names of their own. Apart from the aforementioned fool's mate, these include scholar's mate, smothered mate and the back rank mate.
Traditionally, when checkmate occurs (or is thought to be inevitable) one lays one's king down on its side to indicate that the game has ended.