The legal doctrine that a party may be precluded from denying that certain rights exist if, by behavior or implication that such rights did, in fact, exist, another party has acted upon this information to his or her detriment.
***
The legal principle whereby a person loss the right to deny that a certain condition exists by virtue of his having acted in such a way as to persuade others that the condition does exist. For example, if an insurer allow an insured to violate one of the conditions of the policy, he cannot at a later date void the policy because the condition was violated. The insurer has acted in such a way as to lead the insured to believe that the violation did not void the coverage.
***
UK: The rule of evidence or doctrine of law that precludes a person from denying the truth of some statement formerly made by him, or the existence of facts, which he has by words or conduct led others to believe. Where an insurer continues to deal with a claim despite knowing of a breach giving him the right of avoidance, he may be estopped from denying liability (Evans v. Employers Mutual (1936)).