Repetition acts as an enforcement mechanism: It makes cooperation achievable when it is not achievable in the one-shot game, even when one replaces strategic equilibrium as the criterion for achievability by the more stringent requirement of perfect equilibrium. Robert Aumann
The players in a game are said to be in strategic equilibrium (or simply equilibrium) when their play is mutually optimal: when the actions and plans of each player are rational in the given strategic environment – i. e., when each knows the actions and plans of the others. Robert Aumann