In chess, a pin is a situation brought on by an attacking piece in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece. "To pin" refers to the action of the attacking piece inducing the pin, and 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, i.e., bishops, rooks and queens, can pin opposing pieces. Kings, knights, and pawns cannot pin. Any piece may be pinned except the king, as the king must be immediately removed from check under all circumstances.
"The defensive power of a pinned piece is only imaginary." - Aron Nimzowitsch