Nash equilibrium

A Nash equilibrium is a set of strategies, one for each player, where no one can do better by switching to a different strategy while everyone else keeps theirs the same. It is a general concept from game theory, named after John Nash, and poker's GTO solution is exactly a Nash equilibrium for the two-player zero-sum game a solved spot represents.

Worked example. In rock-paper-scissors, the equilibrium is to play each option a third of the time. Any other mix, say throwing rock more often, gives an opponent who notices a free adjustment: throw paper more and win more. Playing exactly a third of each removes that opening. Poker equilibria work the same way at a much larger scale: a solver searches until every action in every range is priced so that no single deviation, however clever, beats it.

Reaching a true equilibrium in poker is only possible for a solver, since the game tree is too large to balance by hand. What a human takes from the concept is the standard: a strategy is only as safe as the exploits it leaves open.