The notion of a solution concept in game theory. Although the most famous example of such---Nash equilibrium---is rigourously defined, as are several others (correlated equilibrium, rationalizability, sequential equilibrium, etc.), there is no satisfactory general definition of the type of object of which these are tokens. Indeed, the purported definition that appears in this Wikipedia article is, in a sense, as far from informative as it could be without incurring a type mismatch.