(common law) A legal principle in the law of equity that prevents a party from asserting otherwise valid legal rights against another party because of conduct by the first party, or circumstances to which the first party has knowingly contributed, make it unjust for those rights to be asserted.

plural of estoppel

(law) A legal doctrine under which a first party who purports to sell real property that the first party does not actually own to a second party must actually convey that property to the second party if the first party later acquires title to that property.

(law) The estoppel forbidding again litigating an issue decided in a previous case with the same parties, or even with the same party being estopped.

(law) In United States patent law, the circumstance under which a person who has filed a patent application, and then made amendments to the application to accommodate the patent law and secure issuance of the patent, is prevented from bringing a cause of action for infringement of the patent where the allegedly infringing acts falling within the matter disclaimed by the amendment.