GTO (game theory optimal)
GTO (game theory optimal) is the poker strategy that cannot be beaten in the long run heads-up, no matter how an opponent adjusts, because it is balanced against every possible counter.
GTO is shorthand for game theory optimal: the strategy that comes out of solving a poker spot as a two-player zero-sum game. It is the balance point where neither player's range has a profitable adjustment left to make, so an opponent gains nothing by changing how they play against it.
The defining trait of a GTO strategy is balance rather than strength. It bets and bluffs in a mix that stops a caller and a folder from both being right, and it defends widely enough that a bluff cannot automatically profit. That balance is what makes it unexploitable: attack any single part of it and the rest still holds up.
GTO does not promise to win the most money against a specific bad player. It promises a floor: play it well and no opponent, however they adjust, beats you in the long run. Most players study GTO as a baseline, then deviate from it on purpose once they have identified a real, observable leak, since exploiting a mistake is usually more profitable than staying perfectly balanced against it.