Estoppel [from Norman French estouper, to stop up]: A rule of evidence or a rule of law that prevents a person from denying the truth of a statement he has made or from denying facts that he has alleged to exist. The denial must have been acted upon (probably to his disadvantage) by the person who wishes to take advantage of the estoppel or his position must have been altered as a result. There are several varieties of estoppel.