This is a game where one player's gain means the other player's loss. If it is a simple win-lose game, then only one player can win, and eiether the other loses, or they both draw. In a game with scores, it emasn that the total score is fixed. Noughts and crosses is an example of a simple win-lose zero-sum game. Some game playing algorithms such as alpha–beta pruning only work for zero-sum games. This is the opposite of a non-zero-sum game.
Used on page 275