GTO (game theory optimal)
GTO (game theory optimal) is the poker strategy that cannot be beaten in the long run, 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.