A rule of evidence whereby a person is barred from denying the truth of a fact that has already been settled WordNet 3.0 & Open English WordNet

Estoppel is a judicial device in common law legal systems whereby a court may prevent or "estop" a person from making assertions or from going back on his or her word; the person being sanctioned is "estopped". Wikipedia

Judicial device in common law where a court may prevent a person from making assertions Wikidata

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. Wiktionary

Legal principle in the law of equity. Wiktionary (translation)