In a game where two players choose to either cooperate or defect, a Nash equilibrium occurs when:

In a game where two players can choose to cooperate or defect, a Nash equilibrium happens when both players make the best choice they can, given what the other player is doing. This means that neither player can gain an advantage by changing their decision alone. For example, if both players decide to cooperate, they both get a good outcome, but if one player defects while the other cooperates, the defector gets a better result while the cooperator suffers. In this situation, if both players stick to their choices, they reach a point where neither wants to change their strategy, even if it may not lead to the best combined outcome. Thus, a Nash equilibrium reflects a stable situation where players have no incentive to alter their decisions.