Pin
A line attack on a less valuable piece that cannot move without exposing a more valuable piece behind it.
A pin is a tactical motif unique to the long-range pieces — bishop, rook, and queen. A pinning piece attacks an enemy piece along a line, but the real target is the piece behind the pinned one. The pinned piece cannot move because doing so would expose the more valuable target.
Pins come in two kinds. An absolute pin is one where the piece behind is the king: the pinned piece literally cannot move because that would leave the king in check, which is illegal. A relative pin is one where the piece behind is simply more valuable — moving the pinned piece is legal, but it loses material.
The absolute pin is the deadlier of the two because the pinned piece is effectively paralysed. A knight pinned to the king cannot defend other squares, cannot capture, and cannot move at all. Tactical combinations that exploit an absolute pin often begin with another attacker arriving at the pinned piece — since it cannot escape, the attacker simply wins it.
In opening theory the bishop’s pin on a knight is the most common pin in the game. The Ruy Lopez (3.Bb5) and the Italian Game’s quieter lines both use the pin as a positional resource: not to win the pinned piece immediately, but to restrict its movement for many moves and force the opponent to spend tempi on its rescue.