Also known in the US as claim preclusion. A Latin term meaning "a matter judged". This doctrine prevents a party from re-litigating any claim or defence (or issue) already litigated. The doctrine is meant to ensure the finality of judgments and conserve judicial resources by protecting litigants from multiple litigation involving the same claims or issues.

For more information about res judicata, and also the similar concept of abuse of process preventing re-litigation, see: