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The doctrine of res judicata, also known as claim preclusion, prevents a party from re-litigating a claim once a court has issued a final judgment on that claim. A closely related issue, collateral estoppel or issue preclusion, prevents someone from re-litigating a particular issue once a court has ruled on it.

Claim preclusion, or what most courts and commentators would call res judicata, provides that a final, valid judgment on the merits will prevent parties (and those in privity with them) from relitigating the entire claim, i.e., all issues that were or should have been litigated, in a second action.

Issue preclusion, also called collateral estoppel, means that a valid and final judgment binds the plaintiff, defendant, and their privies in subsequent actions on different causes of action between them (or their privies) as to same issues actually litigated and essential to the judgment in the first action.

Generally, claim preclusion applies to counterclaims. Rule 13 of the Federal Rules of Civil Procedure governs counterclaims.If the defendant wins an affirmative defense, then the defendant can counterclaim on the same facts.

Claim Preclusion and Adverse Parties In judicial proceedings, claim preclusion only applies to adverse parties, it does not apply to co-parties (ex: a party that has been joined via Federal Rule of Civil Procedure 19 or Federal Rule of Civil Procedure 20).

Personal and Subject Matter Jurisdiction Unlike res judicata, collateral estoppel does not necessarily require mutuality of parties, meaning that it could affect people or entities that were not part of the original lawsuit, as long as they are within the original court's reach.

Res judicata, also known as claim preclusion, prohibits lawsuits involving the same cause of action and the same parties if the court has entered a final judgment on the merits.