estoppel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A bar that prevents a person from presenting evidence contradicting a certain established fact.

from The Century Dictionary.

• noun Stoppage; impediment.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun The agency by which the law excludes evidence to dispute certain admissions, which the policy of the law treats as indisputable.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a rule of evidence whereby a person is barred from denying the truth of a fact that has already been settled