Pins

Pin - When a piece can not move (either legally or advisedly) because doing so would expose a valuable piece, usually the king or queen, to attack. Pins against the king are called absolute because it is then illegal to move the pinned piece.

Absolute pin - A pin against the king, called absolute because the pinned piece cannot legally move as it would expose the king to check.

Relative pin - A pin, where it is legal to move the pinned piece.

Unpinning - the act of breaking a pin. This allows the piece that was formerly pinned to move.