Es*top"pel (?), n. [From Estop.] (Law) (a) 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. (b) The agency by which the law excludes evidence to dispute certain admissions, which the policy of the law treats as indisputable. Wharton. Stephen. Burrill.

• (law) a legal principle in the law of equity that prevents a party from going back on his or her word.