To estop means to prevent a party from asserting a claim, right, or argument. Courts estop parties from asserting these claims, rights, or arguments through the process of estoppel.
In the field of contract law, promissory estoppel estops parties from claiming that no contract exists when another party was harmed through the reasonable reliance upon a promise by the first party.
In the fields of civil procedure and criminal law, collateral estoppel prevents parties from re-litigating any issue which has already been litigated. Parties are only estopped from re-litigating an issue if that issue was actually, validly, and finally determined on the merits.
[Last updated in October of 2022 by the Wex Definitions Team]