A situation in which a gain by one person or side must be matched by a loss by another person or side.
A game in which the sum of the winnings by all the players is zero. In a zero-sum game, a gain by one player must be matched by a loss by another player. Poker is a zero-sum game if the house does not take a cut as a charge for playing.
A trading situation in which profits are limited and wealth is merely shifted around. The futures and options markets are zero-sum games because for every trader who makes money, another trader loses money.