IN private law estoppel is a many-faceted principle. For example, the doctrine of promissory estoppel in contract law prevents a contracting party, under certain circumstances, from enforcing strict legal rights under the contract because of a promise made not to enforce them; the doctrine of proprietary estoppel in land law disentitles a person (A), under certain circumstances, from denying that another (B), who has acted to his or her (B's) detriment in reliance on a belief that he or she owns certain property, actually has an interest in the property when A has fostered in B the belief that B has the interest. A closely related concept of private law is that of waiver: for example, by his or her conduct a party to a contract may be held to have waived the right to have the contract performed on a particular day. These examples of estoppel and waiver have one basic characteristic in common: they are all cases in which, for some reason, a party is denied the right to assert (or 'is estopped' from asserting) what are admitted to be his or her strict legal rights. So, all cases of estoppel involve a contrast between strict law and some notion of equity which relieves a party of the strict legal consequences defined by a contract or title-deed.
No attempt should be made to draw detailed analogies between any particular type of estoppel or waiver in private law and the doctrine of estoppel in public law. The analogy should be taken no further than the common characteristic noted at the end of the last paragraph: the doctrine of estoppel in public law is primarily concerned with whether a public body ought to be denied the right to rely on and assert the fact that one of its decisions or actions is ultra vires and, therefore, not binding on it. In other words, the primary role of estoppel in public law is to provide a means of creating exceptions to the doctrine of ultra vires.
The language of estoppel is also used in another context. As a general principle, an intra vires decision is binding on the authority which makes it. However, the law does recognize that the demands of public policy can sometimes justify a public authority in changing or revoking