ESTOPPEL, a rule in the law of evidence by which a party in litigation is prohibited from asserting or denying some alleged fact. Estoppel is said to arise in three ways—(I) by record or judgment, (2) by deed, and (3) by matter in pais or conduct. (I) Where a cause of action has been tried and final judgment has been pronounced, the judgment is conclusive—either party attempting to renew the litigation by a new action would be estopped by the judgment. (2) "A man shall always be estopped by his own deed, or not permitted to aver or prove anything in contradiction to what he has once so solemnly and deliberately avowed" (Blackstone, 2 Corn. 295). (3) Estoppel by conduct is the most important head. The rule practically comes to this that, when a person in his dealings with another has so acted as to induce him to believe a thing to be true and to act on such belief, the former may not deny the thing to be true to the detriment of the latter in any proceeding between them; e.g., a partner retiring from a firm without giving notice to the customers, can not, as against a customer having no knowledge of his retirement, deny that he is a partner. As between landlord and tenant the principle operates to prevent the denial by the tenant of the land lord's title. Again, if a man accepts a bill of exchange he may not deny the signature or the capacity of the drawer. So a person receiving goods as baillee from another cannot deny the title of that other to the goods at the time they were entrusted to him. A person cannot, however, be prevented from denying the actuality of what he cannot legally do ; for example, a corporation is not estopped as to acts which would be ultra vires. See EVIDENCE.
See L. F. Everest and E. Strode, The Law of Estoppel (1884) ; M. Cababe, Principles of Estoppel (1888) .