A stable outcome where nobody benefits from changing strategy alone

John Nash proved in 1950 that in games with any number of players, there's always at least one set of strategies where no single player can do better by switching, given what everyone else is doing. It generalised game theory far beyond the simple two-player, zero-sum games studied before him, and became foundational to modern economics, from auction design to antitrust policy. Nash shared the 1994 Nobel Memorial Prize in Economic Sciences for the work.

— John Forbes Nash Jr., Equilibrium Points in N-Person Games — Proceedings of the National Academy of Sciences, 1950