Nash Equilibrium in poker describes a strategy profile where no player has incentive to unilaterally deviate given that all others maintain their strategies. In heads-up zero-sum poker, Nash Equilibrium coincides with the minimax strategy, minimizing maximum loss. Nash’s 1950 theorem guarantees equilibrium existence in finite games, providing the theoretical foundation for all GTO poker theory. In practice, perfect equilibrium is computationally infeasible for full games, but solvers approximate solutions for specific sub-game trees.