Estoppel is an equitable doctrine proposing that any person who asks the courts to enforce a legal remedy should have a clear conscience. Wherever a law is broken, a right of action is created in any person who has suffered a loss (the plaintiff or claimant in the UK). This right to a remedy is enforced through the courts. In almost all cases, the state of the plaintiff's conscience will be irrelevant to obtaining relief. But, if the plaintiff has said or done something that induced the defendant to change his or her behaviour and that reliance was reasonable, the courts have a discretion to deny the remedy to the plaintiff. Hence, an estoppel is not a remedy "at law" in common law jurisdictions, but based on principles of equity. In the majority of cases, it is only a defence and it works by preventing a plaintiff from enforcing established 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 defendant Because its effect is to defeat generally enforceable legal rights, the scope of the remedy is often very limited.
In the case of an existing debt, for example, an estoppel could arise where the creditor informs the debtor that the debt is forgiven, but there is no formal termination of the contract. If the creditor later tries to enforce the original contract terms, but the debtor has relied on the representation and has innocently spent the money on something else, the creditor may be estopped from relying on the usual contractual right to repayment because it would be unfair to allow the creditor to change his or her mind. Similarly, a landlord may tell a tenant that the rent is reduced or cancelled for a specific period of time, e.g. "You can pay half rent until the noise and dirt from the maintenance of the common parts is over." If the tenant changes behavior as a result of what is said, the landlord might be "estopped" from retrospectively claiming the full rent.
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, albeit subtle but important differences exist.