(1):

(n.) The agency by which the law excludes evidence to dispute certain admissions, which the policy of the law treats as indisputable.

(2):

(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.