GTO
Game-theory-optimal: a balanced strategy no opponent can exploit. The unbluffable baseline.
Take a river spot. You're in the big blind with 8♠8♦, the board runs Q♥7♦2♣-5♣-9♦, and the button — who raised preflop with A♣J♣ and missed everything — fires a pot-size bet as a bluff. Your eights beat nothing but a bluff and lose to any pair. A GTO strategy tells you exactly how often to call. A pot-size bluff only profits if it works more than half the time, so you defend half your range: call this hand a fixed fraction of the time, fold the rest. Do that consistently and it stops mattering whether the button holds air or a set. You break even against the bet either way, and he can't exploit you.
GTO doesn't care who's across the table. It's a balanced range and bet-sizing scheme where every action — bet, check, call, bluff, value bet — happens in the ratio that leaves the opponent indifferent between their options, so no adjustment on their end turns a profit. Solvers like PioSOLVER grind out these frequencies hand by hand. Good regulars run GTO as the default when they have no read, then drop it the moment they spot a leak: a player who folds too much gets bluffed more, one who calls too much gets bluffed less.
The beginner mistake is treating GTO as the goal instead of the floor. You memorize a solver's calling frequency, apply it against someone who never bluffs, and pay off bets that should be easy folds. GTO guarantees you won't lose to a perfect opponent. It won't win you the most against an imperfect one — and almost everyone you'll face is imperfect.