What is Check?

A check is a direct attack on the king by an opposing piece or pawn. When a king is in check, the player must immediately respond by (1) moving the king, (2) blocking the check with another piece, or (3) capturing the attacking piece. A player may not make any move that leaves their own king in check. Check is indicated by the symbol "+" in algebraic notation. See also double check, discovered check, and cross-check.