Legal doctrine under which a court can refuse to re-open an issue of fact if it has previously been litigated in another forum. Usually for a court to apply collateral estoppel, it must find that:
- the instant issue is identical to the one decided in the first action;
- that the instant issue must have been actually litigated, (i.e., argued) in the first action;
- a finding on the instant issue was required as part of the final judgment in the first action; and
- the party against whom estoppel would apply had a full and fair opportunity to litigate the issue in the first action.
Collateral estoppel is similar to issue preclusion except that usually the former is at the discretion of the judge while issue preclusion is mandatory, i.e., the issue cannot be reopened. However, it is a different legal concept from equitable estoppel.