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

A Nash equilibrium in a game happens when both players choose their best strategy, given the choice of the other player, and neither player has an incentive to change their decision. In the context of cooperation or defection, if both players decide to cooperate, they might both get a good outcome, but if one defects while the other cooperates, the defector gets a better reward while the cooperator gets a worse one. For example, imagine two friends deciding whether to work together on a project or go solo. If both choose to work together, they achieve a great result, but if one goes solo while the other cooperates, the solo player might get a higher score. Therefore, a Nash equilibrium occurs when both players stick to their choices, knowing that changing their decision would not improve their outcome.