Promissory Estoppel，允诺禁止反言原则，允诺后不得否认原则

Promissory Estoppel：equitable doctrine allowing the court to enforce a promise even though a valid contract was not formed when a person reasonably acted in reliance on that promise. Promissory Estoppel allows the court to compensate the person for their expenditures and/or to avoid the unjust enrichment of the other party.

Promissory Estoppel产生于英美法系国家，它的构成须符合允诺、信赖和公平等要件，并具有暂时性、抗辩性等法律特征。允诺禁止反言原则克服了英美法传统的“约因理论”适用范围狭窄、缺乏灵活性等不足，因而被誉为是法院的一种尝试，以使救济与在整个商业交易中已提高的要诚信表述的良知齐头并进。

参考双语例句：

Promissory Estoppel is a doctrine in Anglo-American law of contracts as the supplement of the theory of consideration.

“允诺禁反言”原则是作为传统的对价理论的对立面而产生的英美合同法概念。

and, lastly, even simple interpretative declarations can be invoked and modified at any time only to the extent that they have not been expressly accepted by the other contracting States or contracting organizations to the treaty or that an estoppel has not been raised against them.

最后，只要单纯的解释性声明没有被条约其他缔约方明确接受，或没有就它们树立一项不容推翻的规定，甚至它们也可以随时被接受和加以更改。