Res judicata or res iudicata (RJ) is the Latin term for “a matter [already] judged”, and may refer to two things: in both civil law and common law legal systems, a case in which there has been a final judgment and is no longer subject to appeal.; and the term is also used to refer to the legal doctrine meant to bar (or preclude) continued litigation of such cases between the same parties, which is different between the two legal systems. In this latter usage, the term is synonymous with “preclusion”.
In the case of RJ, the matter cannot be raised again, either in the same court or in a different court. A court will use RJ to deny reconsideration of a matter.
The legal concept of RJ arose as a method of preventing injustice to the parties of a case supposedly finished, but perhaps mostly to avoid unnecessary waste of resources in the court system. Res judicata does not merely prevent future judgments from contradicting earlier ones, but also prevents litigants from multiplying judgments, so a prevailing plaintiff could not recover damages from the defendant twice for the same injury.