The obligation to move, when any move at all will be bad.

German "forced to move". A position where a player would prefer to pass his move (but of course cannot, as it is illegal) as any move damages his game.

[from German, "compulsion to move"] When a player is put at a disadvantage by having to make a move; where any legal move weakens the position. Zugzwang usually occurs in the endgame, and rarely in the middlegame.