Estoppel is a legal doctrine recognised both at common law and in equity in various forms. It is meant to complement the requirement of consideration in contract law. In general it protects a party who would suffer detriment if:
Unconscionability by the defendant has been accepted as another element by courts, in an attempt to unify the many individual rules of estoppel.
Estoppel is generally only a defense that prevents a representor from enforcing legal rights, or from relying on a set of facts that would give rise to enforceable rights (e.g. words said or actions performed) if that enforcement or reliance would be unfair to the representee. Because its effect is to defeat generally enforceable legal rights, the scope of the remedy is often limited. Note, however, that proprietary estoppel (applicable in English land law) can be both a sword and a shield and the scope of its remedy is wide.
For an example of estoppel, consider the case of a debtor and a creditor. The creditor might unofficially inform the debtor that the debt has been forgiven. Even if the original contract was not terminated, the creditor may be estopped from collecting the debt if he changes his mind later. It would be unfair to allow the creditor to change his mind in light of the unofficial agreement he made with the debtor beforehand. In the same way, a landlord might inform a tenant that rent has been reduced, for example, if there is construction or a lapse in utility services. If the tenant relies on this advice, the landlord could be estopped from collecting rent retroactively.
Estoppel is closely related to the doctrines of waiver, variation, and election and is applied in many areas of law, including insurance, banking, employment, international trade, etc. In English law, the concept of legitimate expectation in the realm of administrative law and judicial review is estoppel's counterpart in public law, although subtle but important differences exist.
This term appears to come from the French estoupail or a variation, which meant "stopper plug", referring to placing a halt on the imbalance of the situation. The term is related to the verb "estop" which comes from the Old French term estopper, meaning "stop up, impede". Note the similarity between the English terms "estop" and "stop".
See venire contra factum proprium in legal systems based on civil law.

The defendant has done or said something to induce an expectation
The plaintiff relied (reasonably) on the expectation...
..and would suffer detriment if that expectation were false. Overview
The main species of estoppel under English, Australian, and American laws are:
Although some authorities regard reliance-based estoppels as mere rules of evidence, they are in reality rules of substantive law.

Estoppel by record This frequently arises as issue/cause of action estoppel or judicial estoppel where the orders or judgments made in previous legal proceedings prevent the parties from relitigating the same issues or causes of action,
Estoppel by deed Where rules of evidence (often regarded as technical or formal estoppels) prevent a litigant from denying the truth of what was said or done, and
Reliance-based estoppels These are the most important forms. Under English law, this class includes estoppel by representation of fact, promissory estoppel and proprietary estoppel (see Halsbury's Laws of England, Vol 16(2), 2003).

• Estoppel by representation of fact is known as equitable estoppel in American law.
  Equitable estoppel as understood in English law, includes:
  
  
  
  • promissory estoppel,
    proprietary estoppel,
    Laches is estoppel by delay. Laches has been considered both a reliance-based estoppel, and a sui generis type of estoppel. Reliance-based estoppels
    In English law, estoppel by representation of fact is a term coined by Spencer Bower. This species of estoppel is also referred to as "common law estoppel by representation" in Halsbury's Laws of England, vol 16(2), 2003 reissue.
    In The Law relating to Estoppel by Representation, 4th edition, 2004 at para I.2.2, Spencer Bower defines estoppel by representation of fact as follows:
    Where one person ('the representor') has made a representation of fact to another person ('the representee') in words or by acts or conduct, or (being under a duty to the representee to speak or act) by silence or inaction, with the intention (actual or presumptive) and with the result of inducing the representee on the faith of such representation to alter his position to his detriment, the representor, in any litigation which may afterwards take place between him and the representee, is estopped, as against the representee, from making, or attempting to establish by evidence, any averment substantially at variance with his former representation, if the representee at the proper time, and in proper manner, objects thereto.
    A second definition can be found at Wilken and Villiers, The Law of Waiver, Variation and Estoppel, 2nd ed, Oxford: 2003, at para 9.02:
    An estoppel by representation [of fact] will arise between A and B if the following elements are made out. First, A makes a false representation of fact to B or to a group of which B was a member. [It is not necessary to demonstrate A knew that the representation was untrue.] Second, in making the representation, A intended or [in the alternatively,] knew that it was likely to be acted upon. Third, B, believing the representation, acts to its detriment in reliance on the representation. [It must have been reasonable to rely on the representation.] Fourth, A subsequently seeks to deny the truth of the representation. Fifth, no defence to the estoppel can be raised by A.
    A representation can be made by words or conduct. Although the representation must be clear and unambiguous, a representation can be inferred from silence where there is a duty to speak or from negligence where a duty of care has arisen. Under English law, estoppel by representation of fact usually acts as a defence, though it may act in support of a cause of action or counterclaim.
    Although there is some debate as to whether "unconscionability" is an element that English courts need to take into account when considering estoppel by representation of fact, the Australian courts clearly do (see Wilken and Villiers, para 9-03; The Commonwealth v Verwayen (1990) 170 CLR 394 at 444 per Deane J.)
    
    Estoppel by representation of fact
    As noted above, although both English and Australian laws treat promissory and proprietary estoppels as species of equitable estoppel, the status of estoppel by representation of fact is less clear in Australia. The decisions of Waltons Stores (Interstate) v Maher (1988) 164 CLR 387 and Commonwealth v Verwayen (1990) 170 CLR 394, both purport to fuse common law and equitable estoppels into a single unified doctrine, but the New South Wales Court of Appeal in Bryon Shire Council v Vaughan [2002] NSWCA 158 continues to treat estoppel by representation at common law as distinct from equitable estoppel. (See Meagher, Gummow & Lehane's Equity: Doctrines & Remedies, 4th edition, Butterworth: 2002, Chapter 17 and Pakinson, The Principles of Equity, 2nd edition, LBC: 2003, Chapter 7). This can be significant in deciding which court has jurisdiction to adjudicate on the issue.
    The American doctrine of equitable estoppel is the same as the English estoppel by representation of fact:
    The most comprehensive definition of equitable estoppel or estoppel in pais is that it is the principle by which a party who knows or should know the truth is absolutely precluded, both at law and in equity, from denying, or asserting the contrary of, any material fact which, by his words or conduct, affirmative or negative, intentionally or through culpable negligence, he has induced another, who was excusably ignorant of the true facts and who had a right to rely upon such words or conduct, to believe and act upon them thereby, as a consequence reasonably to be anticipated, changing his position in such a way that he would suffer injury if such denial or contrary assertion was allowed. 28 Am Jur 2d Estoppel and Waiver § 28
    
    Equitable estoppel
    The traditional version of proprietary estoppel arises in relation to rights to use the land of the owner, and may even be effective in connection with disputed transfers of ownership. So if:
    an estoppel may arise. Thus, in Dillwyn v Llwellyn (1862) 4 De G.F.& J. 517 C.A. a father promised a house to his son who took possession and spent a large sum of money improving the property. The father never actually transferred the house to the son. When his father died, the son claimed to be the equitable owner and the court ordered the testamentary trustees to convey the land to him. See also Inwards v Baker [1965] 2 Q.B. 29, C.A.
    In Wilmott v Barber (1880) 15 Ch D 96, Fry J considered that five elements had to be established before proprietary estoppel could operate:
    Although proprietary estoppel was only traditionally available in disputes affecting title to real property, it has now gained limited acceptance in other areas of law. Proprietary estoppel is closely related to the doctrine of constructive trust.
    The term "proprietary estoppel" is not used in American law, but is part and parcel of the general doctrine of promissory estoppel. In English law, proprietary estoppel is distinct from promissory estoppel.
    
    one party represents that he or she is transferring an interest in land to another, but what is done has no legal effect, or
    merely promises at some time in the future to transfer land or an interest in land to another, and
    knows that the other party will spend money or otherwise act to his or her detriment in reliance on the supposed or promised transfer,
    the plaintiff must have made a mistake as to his legal rights;
    the plaintiff must have done some act of reliance;
    the defendant, the possessor of a legal right, must know of the existence of his own right which is inconsistent with the right claimed by the plaintiff;
    the defendant must know of the plaintiff's mistaken belief; and
    the defendant must have encouraged the plaintiff in his act of reliance. Proprietary estoppel
    The doctrine of promissory estoppel prevents one party from withdrawing a promise made to a second party if the latter has reasonably relied on that promise and acted upon it.
    
    Promissory estoppel
    
    
    Main article: Estoppel (English law) English law
    The doctrine of promissory estoppel was adopted into Australian law in Legione v. Hateley (1983) 152 CLR 406; however, the plaintiffs were unsuccessful in that case because the reliance was unreasonable and the promise not unequivocal.
    In fact, now Australian law has gone beyond the position espoused in the High Trees case; it has been extended successfully to cases where there is no pre-existing legal relationship between the two parties, and promissory estoppel can be wielded as a "sword", not just as a "shield". Mason CJ and Wilson J in Waltons Stores (Interstate) Ltd v. Maher (1988) 164 CLR 387 held that if estoppel is proven, it gives rise to an equity in favour of the plaintiff, and the court will do the minimum equity that is just in the circumstances. From this case, it is also possible for the promise to come from silence or inaction.
    As noted above, in Australian law, there is an element of unconscionability, which is satisfied if one party encourages the other party to create assumptions that lead to reliance.
    Today, the principle of estoppel may give birth to an enforceable obligation even without a consideration under the following conditions: 1. promise 2. dishonest behavior of the promittant 3. special relationship between the promittant and the beneficior (eg: duty of information) 4. irreversible changement of the situation of the beneficior of the promise
    
    Australian law
    In the many jurisdictions of the United States, promissory estoppel is generally an alternative to consideration as a basis for enforcing a promise. It is also sometimes referred to as detrimental reliance.
    The American Law Institute in 1932 included the principle of estoppel into § 90 of the Restatement of Contracts, stating:
    A promise which the promisor should reasonably expect to induce action or forbearance of a definite and substantial character on the part of the promisee and which does induce such action or forbearance is binding if injustice can be avoided only by enforcement of the promise. Restatement (Second) removed the requirement that the detriment be "substantial".
    The distinction between promissory estoppel and equitable estoppel should be noted:
    Equitable estoppel is distinct from promissory estoppel. Promissory estoppel involves a clear and definite promise, while equitable estoppel involves only representations and inducements. The representations at issue in promissory estoppel go to future intent, while equitable estoppel involves statement of past or present fact. It is also said that equitable estoppel lies in tort, while promissory estoppel lies in contract. The major distinction between equitable estoppel and promissory estoppel is that the former is available only as a defense, while promissory estoppel can be used as the basis of a cause of action for damages. 28 Am Jur 2d Estoppel and Waiver § 35
    Suppose that B goes to a store and sees a sign that the price of a radio is $10. B tells the shopkeeper that he will get the money and come back later that day to purchase it; there is no discussion of price. The shopkeeper says that when B returns, he will be happy to deal with B as he deals with all his customers but that, if he sells all the radios (he has three), he will not be able to help B. Hearing this, B goes and sells his watch for $10 (it was really worth $15, but since B wanted the money right away, he chose not to wait for the best price). When B returns, the sign says $11, and the owner tells B that he has raised the price. In Equity, can you argue that the shopkeeper is estopped by conduct? B relied upon the implied representation that a radio would be sold for $10 when he returned with the money; B has sold his watch at a price lower than the market price, and thus he has acted to his detriment. (Note that if B's watch was worth $10, and he received a fair price, there would be no detriment.) But the problem is that the shopkeeper did not guarantee to hold one of the radios against the possibility of B's return nor did they agree a fixed price. The shopkeeper's conscience might have been affected if he had known that B was going home to collect the money and would definitely return to buy one of the three radios. Indeed, in some common law jurisdictions, a promise by the shopkeeper to hold a specific radio would create a binding contract, even if B had to go for the money. A promise to pay the owner in the future is good consideration if it is made in exchange for a promise to sell a specific radio (one from three is probably sufficiently specific): one promise in exchange for a second promise creates equal value. So the shopkeeper's actual words and knowledge are critical to deciding whether either a contract or an estoppel arises.
    For an example of promissory estoppel in the construction industry, suppose that B Ltd consolidates estimates from a number of subcontractors and quotes a single price on a competitive tender. The client accepts B Ltd's quote and construction begins. But one of the subcontractors then claims reimbursement above its original estimate and, because of this change, B Ltd cannot profit from the works. If both parties knew that the accuracy of the individual estimates was critical to the success of the tender and the profitability of the contract as a whole, a court might apply promissory estoppel and allow B Ltd to pay only what the subcontractor originally estimated rather than the new, higher price. But, if both parties hoped that there would be an opportunity to increase the contract prices to reflect additional expenditure, the subcontractor's conscience would not be as limited in seeking a higher payment and B Ltd might be penalised for not building an adequate contingency sum into the tendered price.
    One contentious point during the drafting of the Restatement was how to calculate the amount of damages flowing from a promissory estoppel. During the deliberations, the following example was considered: a young man's uncle promises to give him $1,000 to buy a car. The young man buys a car for $500, but the uncle refuses to pay any money. One view was that the young man should be entitled to $1,000 (the amount promised), but many believed that the young man should only be entitled to $500 (the amount he actually lost). The language eventually adopted for the Second Restatement reads: "The remedy granted for breach may be limited as justice requires." — a formula which leaves quantification to the discretion of the court.
    
    American law
    
    Other estoppels
    Estoppel in pais (literally "by act of notoriety", or "solemn formal act") is the historical root of common law estoppel by representation and equitable estoppel. The terms Estoppel in pais and equitable estoppel are used interchangeably in American law.
    
    Estoppel in pais
    Estoppel by convention in English law (also known as estoppel by agreement) occurs where two parties negotiate or operate a contract but make a mistake. If they share an assumption, belief or understanding of how the contract will be interpreted or what the legal effect will be, they are bound by that belief, assumption or understanding if:
    Some say that that estoppel by convention is not truly an estoppel in its own right, but merely an instance of reliance-based estoppel (estoppel by representation would be its most frequent form). Others see it as no more than an application of the rule of interpretation that, where words in a contract are ambiguous, you always interpret those words so as to give effect to the actual intentions of the parties even though that would not be the usual legal outcome.
    
    (i) they both knew the other had the same belief, and
    (ii) they both based their subsequent dealings on those beliefs. Estoppel by convention
    Estoppel by acquiescence may arise when one person gives a legal warning to another based on some clearly asserted facts or legal principle, and the other does not respond within "a reasonable period of time". By acquiescing, the other person is generally considered to have lost the legal right to assert the contrary.
    As an example, suppose that Jill has been storing her car on Jack's land with no contract between them. Jack sends a registered letter to Jill's legal address, stating: "I am no longer willing to allow your car to stay here for free. Please come get your car, or make arrangements to pay me rent for storing it. If you do not do so, within 30 days, I will consider the car abandoned and will claim ownership of it. If you need more time to make arrangements, please contact me within 30 days, and we can work something out." If Jill does not respond, she may be said to have relinquished her ownership of the car, and estoppel by acquiescence may prevent any court from invalidating Jack's actions of registering the car in his name and using it as his own.
    
    Estoppel by acquiescence
    Estoppel by deed is a rule of evidence arising from the status of a contract signed under seal — such agreements, called deeds, are more strictly enforced than ordinary contracts and the parties are expected to take greater care to verify the contents before signing them. Hence, once signed, all statements of fact (usually found in the opening recital which sets out the reason(s) for making the deed) are conclusive evidence against the parties who are estopped from asserting otherwise.
    
    Issue estoppel
    
    Judicial estoppel
    Assignor estoppel
    Collateral estoppel (USA)
    Prosecution history estoppel also known as file-wrapper estoppel (USA)
    De facto corporation and corporation by estoppel (USA)
    Estoppel by deed (USA)
    Acquiescence and Estoppel by acquiescence
    Punishment and Proportionality: The Estoppel Approach, by Stephan Kinsella, a rights theory based on the concept of estoppel