Folk Theorem
The game-theoretic result, so named because it emerged gradually from shared understanding rather than one author's proof, stating that in a repeated game with a long enough and certain enough shadow of the future, a very wide range of stable outcomes can be sustained between players — not just the single outcome a one-shot version of the game would predict. The folk theorem is agnostic about whether the sustained outcome is desirable: it explains both honest reputation markets and price-fixing cartels with the identical logic.