Estoppel is a judicial device whereby a court may prevent or "estop" a person from making assertions or from going back on their word. The person barred from doing so is said to be "estopped". Estoppel may prevent someone from bringing a particular claim. In common law legal systems, the legal doctrine of estoppel is based in both common law and equity.

Promissory and proprietary estoppel are specific forms of estoppel reflecting distinct legal traditions within various jurisdictions. Estoppel is also a concept in international law.

There are many different types of estoppel, but the common thread between them is that a person is restrained from asserting a particular position in law where it would be inequitable to do so. By way of illustration:

Estoppel is traditionally an equitable doctrine. Accordingly, it is sometimes said that any person wishing to assert an estoppel must come to the court with "clean hands".

The doctrine of estoppel (which may prevent a party from asserting a right) is often confused with the doctrine of waiver (which relates to relinquishing a right once it has arisen). It also substantially overlaps with, but is distinct from, the equitable doctrine of laches.

"Estop" is a verb of Anglo-Norman origin meaning "to seal up", while the noun "estoppel" is based on Old French estoupail (stopper). When a court finds that a party has done something warranting a form of estoppel, that party is said to be estopped from making certain related arguments or claiming certain related rights. The defendant is estopped from presenting the related defense, or the plaintiff is estopped from making the related argument against the defendant. Lord Coke stated, "It is called an estoppel or conclusion, because a man's own act or acceptance stoppeth or closeth up his mouth to allege or plead the truth."

Estoppel is sometimes said to be a rule of evidence whereby a person is barred from leading evidence of a fact that has already been settled or they are otherwise precluded from asserting, but that may be an oversimplification. Firstly, although some estoppels relate to preventing a party from asserting facts, others relate to preventing a party from asserting a right or a claim. Secondly, under the conflict of laws in common law jurisdictions, matters of evidence are usually treated as procedural matters for the law of the local court (the lex fori), whereas it is generally accepted that an estoppel may affect substantive rights and are therefore matters to be determined by the proper law (or lex causae) that governs the particular issue.

There are many different types of estoppel that can arise under common law legal systems. It has been judicially noted on more than one occasion that the link between them is often somewhat tenuous. Treitel on Contracts notes that "unconscionability ... provides the link between them", but they nevertheless have "separate requirements and different terrains of application". The courts have long abandoned an attempt to create a single general underlying rationale or principle: