PIN

At attack by a queen, rook or bishop on a piece which cannot move without exposing a more important piece or square. The pinned piece is said to be masking or screening the more important piece or square. An absolute pin is where the screened piece is a king, therefore it is illegal for the pinned piece to move as it would expose the king to check. A relative pin is where moving the pinned piece would result in a loss of material or other unfavourable effects.