Promissory estoppel is a legal principle that is used to enforce agreements not normally binding.
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- ...it is the first principle upon which all Courts of Equity proceed, that if parties who have entered into definite and disctinct terms involving certain legal results — certain penalties or legal forfeiture — afterwards by their own act or with their own consent enter upon a course of negotiation which has the effect of leading one of the parties to suppose that the strict rights arising under the contact will not be enforced, or will be kept in suspense, or held in abeyance, the person who otherwise might have enforced those rights will not be allowed to enforce them where it would be inequitable having regard to the dealings which are thus taken place between the parties.
- Lord Cairnes in Hughes v. Metropolitan Railway Co. 
- The principle, as I understand it, is that whether one party has, by his words or conduct, made to the other a promise or assurance which was intended to affect the legal relations between them and to be acted on accordingly, then, once the other party has taken him at his word and acted on it, the one who gave the promise or assurance cannot afterwards be allowed to revert to the previous legal relations as if no such promise or assurance had been made by him, but he must accept their legal relations subject to the qualification which he himself has so introduced, even though it is not supported in point of law by any consideration, but only by his words.
- Lord Denning in Combe v. Combe 
- A promise which the promisor should reasonably expect to induce action or forbearance on the part of the promisee or a third person and which does induce such action or forbearance is binding if injustice can be avoided only by enforcement of the promise.