Zugzwang
A position where every legal move worsens your position — but you must move anyway.
Zugzwang — German for “compulsion to move” — describes the dilemma at the heart of many endgames. The player whose turn it is would prefer to pass. Every legal move makes the position worse. But chess does not allow passing, so a move must be made, and the worse position is the result.
Zugzwang is most common in king-and-pawn endgames. The opposition — kings on the same file with one square between them — is the classical zugzwang position. Whichever king must move first loses ground. The side not to move is said to have the opposition and effectively controls the position.
Knight endgames also feature zugzwang. The knight is unique among pieces in having no way to make a “passing” move — it must always move to a different square of different color. In some configurations the knight has no useful square to go to, and every knight move loses something.
Mutual zugzwang is the rare and elegant case where both sides are in zugzwang — whoever moves loses. These positions feature in endgame studies and in the deepest theoretical endgames; they require precise tempo counting to engineer or to avoid.
The opposite of zugzwang — having a move that improves the position — is called the move or the initiative. In most middlegames, having the move is an advantage. In specific endgame positions, having the move is a fatal liability.