禁止反言原则(equitable estoppel)是英美法系国家的一般契约理论，其基本内涵是“My word is my bond”——言行一致，不得出尔反尔。英美法系国家传统的契约理论是：合同成立、变更均须有对价(consideration)，才能产生强制执行之效力。可是实践中大量存在着这样的现象：某人许诺赠与他人物品或答应他人无偿为其做某事，但不久又反悔而使受诺人遭受损失。受诺人受到损失后却无法律依据阻却权利人权利的行使。20世纪40年代以前，英美法系国家的法官解决这一问题的方法，使法律天平平衡的砝码，一是在衡平法上采用禁反言，再是尽其所能寻找有效的约因。这种方法往往产生牵强附会，徒增了人们对法律公正性的怀疑。1877年英国法官卡恩斯勋爵(Lord Cairns) 审理 Hughes V .Metropolitan Railway Co.案时就提出了禁止反言的观念，但并未引起人们的重视。直至1947年，卡恩斯勋爵的观念被英国大法官丹宁(Lord Denning)传承，并将其确立为一个法律原则。

Estoppel is a collective name given to a group of legal doctrines in common law legal systems whereby a person is prevented from asserting certain matters before the court to prevent injustice - the person is said to be "estopped". Estoppel may operate by way of preventing someone from asserting a particular fact in court, or exercising a certain right, or from bringing a particular claim. Black's Law Dictionary defines estoppel as a "bar or impediment raised by the law, which precludes a man from alleging or from denying a certain fact or state of facts, in consequence of his previous allegation or denial or conduct or admission, or in consequence of a final adjudication of the matter in a court of law."