A skewer is a tactic in chess, somewhat similar to the pin. Whereas with a pin you have a lower-valued piece standing in the way of an attack on a higher-valued piece, the skewer reverses the pieces' roles: a valuable piece is attacked, forcing it to move out of the way and thereby expose an attack on a less valuable piece. For example, if a column of the board contains, from top to bottom, a black rook, white king and white queen, then the king is forced to move out of check leaving the queen to be captured by the rook.

An example game containing a skewer:

1. e3 e5 
2. f4 d6 
3. Nh3 Qh4+ 
4. Ke2 Bg4+
The black bishop skewers the white king and will capture white's queen on the next move.