Is a strategy profile such that every player's strategy is a best response to the strategies of all the other players. In a Nash Equilibrium no player can unilaterally deviate to obtain more profits. However there are some games in which have either no Nash Equilibrium or multiple nash equilibria.
Below is a 2x2 payoff matrix and the equilibrium concept to this game. In this instance both players are playing their best response at (Defect, Defect). For more information on this game and how it's solved see the Prisoners' Dilemma Game.
