res judicata

Res judicata is a Latin phrase meaning 'the adjudicated thing' or 'the matter decided,' formed from res (thing, matter, affair) and judicata, the feminine past participle of judicare (to judge, to decide), agreeing with res in gender and case. The phrase names one of the oldest and most fundamental principles of legal systems across the world: that a final judgment by a court of competent jurisdiction resolves the dispute between the parties once and for all, and that neither party may relitigate the same matter in a subsequent proceeding. Without this principle, litigation would never end — a losing party could simply file a new suit raising the same issues.

The principle of res judicata has roots in Roman law, where the concept of the 'litis contestatio' (the joinder of issue in a Roman action) had a similar effect of consuming and extinguishing the original claim once proceedings were underway. The Digest of Justinian, compiled in 533 CE, contains extensive discussion of the effects of judgments and the binding nature of prior decisions. Roman law developed two related doctrines: one barred a plaintiff from bringing the same claim twice (the claim preclusion aspect), and another prevented relitigation of issues that had been actually determined (the issue preclusion aspect). Both are encompassed in the modern doctrine of res judicata.

English common law developed parallel doctrines under different names — 'estoppel by record,' 'merger,' and 'bar' — but the Latin phrase res judicata came to serve as the umbrella term encompassing these concepts. The doctrine has two main branches in American law: claim preclusion (traditionally called res judicata proper), which bars relitigation of claims that were or could have been raised in prior litigation; and issue preclusion (traditionally called collateral estoppel), which bars relitigation of specific issues that were actually litigated and necessarily decided. Both branches protect the finality of judgments and the integrity of the judicial system.

The importance of res judicata extends beyond individual cases to the stability of legal relationships generally. Property titles, contract rights, and family status determinations all rest on the assumption that a judgment settling them will be respected. A world without res judicata would be one in which no legal question was ever finally answered — each adverse ruling could be endlessly appealed and the original action refiled until a sympathetic court was found. The doctrine's effect on individual cases can seem harsh, particularly when the original judgment was wrong, but the alternative — endless litigation — is worse for the legal system as a whole.