Nash equilibrium
Also said as Game theory optimal, GTO, Mixed strategy
A set of strategies where nobody can do better by changing theirs alone. Often it means mixing your choices in exact proportions, so that the other side can't exploit you whatever they do.
It shows you where a game settles once players have worked it out. If the settling point is dull, or one option is never used in it, that option is broken. Being at it means nobody can punish you. It does not mean you are punishing them.
Say it
“Run the match-ups through a solver. If the equilibrium uses the dagger under 2% of the time, the dagger is too weak. I don't care that some players enjoy it.”
Where you’ve seen it
In 2015, researchers at the University of Alberta announced that their program Cepheus had essentially solved heads-up limit Texas hold'em, working out a strategy so near to unbeatable that a lifetime of play could not reliably show the difference.
Not to be confused with dominant strategy
A dominant strategy is best no matter what the other side does. An equilibrium is only stable: each side is doing the best it can, given what the other is doing.