A rule of law that when person 'A', by act or words, gives person 'B' reason to believe a certain set of facts upon which person 'B' takes action, person 'A' cannot later, to his (or her) benefit, deny those facts or say that his (or her) earlier act was improper.
Estoppel is a set of doctrines in which a court prevents a litigant from taking an action the litigant normally would have the right to take, in order to prevent an inequitable result. Estoppel occurs when a party "reasonably relies on the promise of another party, and because of the reliance is injured or damaged". For example, estoppel precludes "a person from denying, or asserting anything to the contrary of, that which has, in contemplation of law, been established as the truth, either by the acts of judicial or legislative officers, or by his own deed, acts, or representations, either express or implied".
Estoppel is the law's way of saying "you can't have your cake and eat it." One British judge said, in 1862: