What is "collateral estoppel"

n. The estoppel forbidding again litigating an issue decided in a previous case with the same party, or even with the same party being estopped.

Collateral estoppel (CE), known in modern terminology as issue preclusion, is a common law estoppel doctrine that prevents a person from relitigating an issue. One summary is that, "once a court has decided an issue of fact or law necessary to its judgment, that decision ... preclude[s] relitigation of the issue in a suit on a different cause of action involving a party to the first case". The rationale behind issue preclusion is the prevention of legal harassment and the prevention of abuse of judicial resources.