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. A player is said to be "in zugzwang" when any possible move will worsen his position.
The term is also used in combinatorial game theory, where it means that it directly changes the outcome of the game from a win to a loss, but the term is used less precisely in games such as chess. Putting the opponent in zugzwang is a common way to help the superior side win a game, and in some cases, it is necessary in order to make the win possible.
The term "zugzwang" was used in German chess literature in 1858 or earlier, and the first known use of the term in English was by World Champion Emanuel Lasker in 1905. The concept of zugzwang was known to players many centuries before the term was coined, appearing in an endgame study published in 1604 by Alessandro Salvio, one of the first writers on the game, and in shatranj studies dating back to the early 9th century, over 1000 years before the first known use of the term. 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: