Source : Webster's Revised Unabridged Dictionary (1913)

Estoppel \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.

Source : WordNet®

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