estoppel

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Law A bar preventing one from making an allegation or a denial that contradicts what one has previously stated as the truth.

Century Dictionary and Cyclopedia

1. n. Stoppage; impediment.
2. n. In law, the stopping of a person by the law from asserting a fact or claim, irrespective of its truth, by reason of a previous representation, act, or adjudication inconsistent therewith.

Wiktionary

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

GNU Webster's 1913

1. n. A stop; an obstruction or bar to one's alleging or denying a fact contrary to his own previous action, allegation, or denial; an admission, by words or conduct, which induces another to purchase rights, against which the party making such admission can not take a position inconsistent with the admission.
2. n. The agency by which the law excludes evidence to dispute certain admissions, which the policy of the law treats as indisputable.

WordNet 3.0

1. n. a rule of evidence whereby a person is barred from denying the truth of a fact that has already been settled