What is Res Judicata?

Legal Definition
Res judicata or res iudicata, also known as claim preclusion, is the Latin term for "a matter [already] judged", 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 is no longer subject to appeal; and the legal doctrine meant to bar (or preclude) continued litigation of a case on same issues between the same parties. In this latter usage, the term is synonymous with "preclusion".

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 legal concept of res judicata 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, and confusion.
-- Wikipedia
Legal Definition
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.)
But see:
Legal Definition
Latin- a thing adjudicated. Once a lawsuit is decided, the same issue or an issue arising from the first issue cannot be contested again.
Legal Definition
A matter adjudged; a thing judicially acted upon or decided; a thing or matter settled by judgment. A phrase of the civil law, constantly quoted in the books. 2 Kent, Comm. 120.
-- Black's Law Dictionary
Legal Definition
Same as Res adjudicata.
-- Ballentine's Law Dictionary