Coase Theorem
If trade in an externality is possible and there are sufficiently low transaction costs, bargaining will lead to a Pareto efficient outcome regardless of the initial allocation of property.
Origin
Ronald Coase published "The Problem of Social Cost" in the Journal of Law and Economics in 1960, examining how legal liability rules interact with market externalities; it became the most-cited law review article in history. Yet Coase never called his insight a theorem — that name was applied by George Stigler in the 1966 edition of his textbook The Theory of Price. Coase received the Nobel Prize in Economic Sciences in 1991; scholars have since noted that Stigler's framing emphasised the zero-transaction-cost thought experiment over Coase's actual argument that transaction costs do matter.