A tactical situation in which an piece holds down an enemy piece. There are two types of pins:
1) absolute pin;
2) relative pin.
In the former case it is illegal for the pinned piece to make a move, in the latter case making a move is punished by a substantial loss of material (often a Queen or a Rook). One should be careful with relative pins as they can be broken by tactical means.
Example of an absolute pin: 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 (the Knight cannot be moved).
Example of a relative pin: 1.e4 e5 2.Nf3 d6 3.Bc4 Bg4 (the Knight can be moved but then a Queen is lost). A nice example of the dangers of this type of pin is given by the possible continuation: 4.Nc3 Nc6 5.0-0 Nd4? 6.Nxe5!! (the pin is deliberately broken) Bxd1 7.Bxf7+ Ke7 8.Nd5#.