Solutio indebiti

In the civil law. Payment of what was not due. From the payment of what was not due arises an obligation quasi ex contractu. When one has erroneously given or performed something to or for another, for which he was in no wise bound, he may redemand it, as if he had only lent it. The term "solutio indebiti" is here used in a very wide sense, and includes also the case where one performed labor for another, or assumed to pay a , debt for which he was not bound, or relinquished a right or released a debt, under the impression that he was legally bound to do so. Mackeld. Rom. Law, § 500.

Defined under Solutio in Black's Law Dictionary.