Res judicata (RJ) or res iudicata, also known as claim preclusion, is the Latin term for matter decided and refers to either of two concepts in both civil law and common law legal systems: a case in which there has been a final judgment and that is no longer subject to appeal; and the legal doctrine meant to bar (or preclude) relitigation of a claim between the same parties.

In the case of res judicata, the matter cannot be raised again, either in the same court or in a different court. A court will use res judicata to deny reconsideration of a matter.

The doctrine of res judicata is a method of preventing injustice to the parties of a case supposedly finished but perhaps also or mostly a way of avoiding unnecessary waste of judicial resources. Res judicata does not merely prevent future judgments from contradicting earlier ones, but also prevents litigants from multiplying judgments, and confusion.