An attack on the King. You do not have to announce “check”.

The act of attacking the opponent's king. When check takes place, a player usually calls out "check" so the opponent is aware of the threat. The opponent must get out of check on the next move, either by moving the King, capturing the attacking piece, or moving another piece between the King and the attacking piece.

A direct attack on the king by an enemy man. The attacked king is said to be in check. There are only three possible immediate responses to a check: capturing the attacking piece, moving the king to an unattacked square, or interposing a piece between the attacker and the king. In casual games a player usually announces "check", however this is not a requirement in tournament games.