Opposition is an endgame technique in which two kings face each other on the same rank or file, separated by an odd number of squares, with the player whose turn it is to move forced to give ground.
The player who does not have to move is said to "have the opposition": their opponent must move the king first, conceding key squares or a path for the other king to advance. The most common form is direct opposition: both kings separated by a single square on the same rank or file. Distant opposition works on the same principle but with three or five squares between the kings.
In practice, mastering the opposition is essential in King and Pawn versus King endgames. It often makes the difference between promoting a pawn and conceding a draw by stalemate or positional deadlock. Before pushing your pawn, always ask yourself: "Does my king have the opposition?"