The central concept of noncooperative game theory is that of the strategic equilibrium (or Nash equilibrium, or noncooperative equilibrium). A strategic equilibrium is a prole of strategies or plans, one for each player, such that each player's strategy is optimal for him, given the strategies of the others.
In most of the early literature the idea of equilibrium was that it said something about how players would play the game or about how a game theorist might recommend that they play the game. More recently, led by Harsanyi (1973) and Aumann (1987a), there has been a shift to thinking of equilibria as representing not recommendations to players of how to play the game but rather the expectations of the others as to how a player will play. Further, if the players all have the same expectations about the play of the other players we could as well think of an outside observer having the same information about the players as they have about each other.