What is Pin (chess)?

In chess, a pin is a chess tactic in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece. Moving the attacking piece to bring on the pin is called pinning; the defending piece so restricted is described as pinned. Only pieces that can move an indefinite number of squares in a horizontal, vertical, or diagonal line can pin. Kings, knights, and pawns cannot pin. Any piece can be pinned except the king, since the king must be immediately removed from check under all circumstances. It is like a skewer, but in a skewer, the more valuable piece is the one under direct attack.