Definition:Chess/Rules/In Check

Definition
Uniquely among the chess pieces the king may not legally move into a square which is under attack by a piece of the opposite colour.

That is, a king may not be moved so as to placed under attack.

If player $a$ moves a piece so as to place the king of player $b$ under attack, then player $b$'s king is said to have been placed in check.

Player $a$ is then obliged to say "Check."

Player $b$ must make a move so that his or her king is no longer in check.

Such a move by player $b$ is known as getting out of check.