zero-sum game

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A situation in which a gain by one person or side must be matched by a loss by another person or side.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A game (or a similar economic system) in which the winnings of some players must equal the losses of others.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a game in which the total of all the gains and losses is zero