Pronounced "tsoog-tsvung", Zugzwang is a German word meaning "obligation to move". The term is used for a position in which whoever has the move would obtain a worse result than if it were the opponent’s turn to play. The term was first used in a German chess magazine in 1858. Below is the Immortal Zugzwang game, where in the final position, any move Black makes, loses quickly.
Whoever is to move in the following diagram IMMEDIATELY loses. This is because they must not only cease attacking the opponent's pawn, but must give up defense of their own! This situation is refferd to as a 'trebuchet'.
White to move:
Black to move: