Literally "a matter judged", res judicata is the principle that a matter may not, generally, be relitigated once it has been judged on the merits.
Res judicata encompasses limits on both the claims and the issues that may be raised in subsequent proceedings:
- Claim preclusion is the principle once a cause of action has been litigated, it may not be relitigated.
- Bar: A losing plaintiff is barred from re-suing a winning defendant on the same cause of action. (Scenario: Plaintiff P unsuccessfully sues Defendant D on Cause of action C. P may not try for better luck by initiating a new lawsuit against D on C.)
- Merger: A winning plaintiff may not re-sue a losing defendant. (Scenario: P successfully sues D on C. P may not again sue D on C to try to recover more damages.)
- Issue preclusion (Collateral estoppel): Once an issue of fact has been determined in a proceeding between two parties, the parties may not relitigate that issue even in a proceeding on a different cause of action. (Scenario: P sues D on C. P sues D on C1. Element E, which was determined in the first trial, is common to C and C1. At the second trial, P and D cannot attempt to get a different disposition of E.)