The Collaborative International Dictionary of English v.0.48:

game theory \game theory\ n.
   A branch of mathematics that deals with strategies for
   maximizing gains or minimizing losses in competitive
   situations having defined constraints and involving random
   factors.

   Note: Game theory is used for modelling and analysis of
         various decision-making situations such as military
         strategy or business policy. The theoretical models
         study the interactions among opposing entities called
         "players," where different kinds of situation can
         arise, for which the probabilities of occurence are
         known. Also known is the set of decisions each player
         can take. When a player takes a decision he makes a
         gain or incurs a loss. Based on the available knowledge
         each player tries to adopt a "strategy" so as to
         maximize his gains. The entire procedure constitutes a
         game.

   Syn: theory of games.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

game theory
    n 1: (economics) a theory of competition stated in terms of
         gains and losses among opposing players [syn: game
         theory, theory of games]