judicata

Judicata, commonly referred to in its fuller form as res judicata, is a legal principle that prevents the relitigation of claims or issues that have been finally adjudicated by a competent court. The term comes from Latin and means "a thing judged."

Res judicata encompasses two related preclusion doctrines: claim preclusion and issue preclusion. Claim preclusion bars a party from presenting in a later action the same claim that was or could have been raised in a prior suit. Issue preclusion, also known as collateral estoppel, prevents relitigation of particular factual or legal issues that were actually litigated and essential to the prior judgment, in subsequent proceedings.

For res judicata to apply, courts generally require several elements: a final judgment on the merits; a court of competent jurisdiction; identity of parties or their privies; and the same cause of action or claim in the later action. In many jurisdictions, there is also a requirement that the party had a full and fair opportunity to litigate the matter, and that the judgment is valid and not obtained through fraud or a denial of due process.

When these requirements are met, the later action on the same claim is barred. However, exceptions exist, including judgments obtained by fraud, lack of jurisdiction, or fundamental defects, and variations in how different jurisdictions determine the scope of preclusion. Some jurisdictions also recognize limits based on new facts, new legal theories, or changes in law that affect the claim.

Judicata is a central concept in civil procedure and is applied differently across common law jurisdictions. It is related to, but distinct from, collateral estoppel in some systems and to other forms of preclusion such as double jeopardy in others.