Collateral estoppel, more commonly called issue preclusion today, prevents a party from relitigating a specific factual or legal issue that was already actually litigated and necessarily decided in a prior lawsuit -- even when the second lawsuit asserts a completely different claim. It is narrower than its sibling doctrine, res judicata (claim preclusion), which bars an entire cause of action from being relitigated between the same parties; collateral estoppel operates issue-by-issue and can carry over into a later case built on an unrelated legal theory.

Traditionally, the doctrine required mutuality -- both sides in the second case had to be the same parties, or their privies, as in the first. Most modern courts, following the Supreme Court's guidance in cases like Parklane Hosiery, now allow non-mutual collateral estoppel, letting a party who was not involved in the first suit invoke its outcome against someone who did litigate and lose the issue there. Courts scrutinize the offensive use of this rule more closely than the defensive use.

A party invoking collateral estoppel must generally show that the issue in the second case is identical to one raised in the first, that the issue was actually litigated, that it was actually and necessarily decided -- meaning it was essential to the judgment rather than incidental commentary or dicta -- and that the party being bound had a full and fair opportunity to litigate it the first time. A final, valid judgment on the merits in the prior action is also required.

Issue preclusion differs from claim preclusion in scope: claim preclusion bars an entire cause of action between the same parties, while issue preclusion can bind a single determination even across different claims. Within non-mutual issue preclusion, courts distinguish defensive use, where a new defendant simply invokes a plaintiff's earlier loss, from offensive use, where a new plaintiff invokes a defendant's earlier loss -- and apply extra caution to offensive use, weighing whether the plaintiff could have joined the earlier suit and whether the defendant had a full incentive to litigate it vigorously.

The party asserting preclusion produces the prior judgment, pleadings, and record to show the issue was identical, actually litigated, and necessary to that outcome. The opposing party attacks by arguing the issue differs in some material respect, was not essential to the earlier judgment, or that the earlier proceeding did not afford a full and fair opportunity to litigate -- for example, due to limited discovery or a lower court's jurisdictional damages cap. Juricratic can model unresolved preclusion questions as a dial affecting the simulated range of available defenses.