Res judicata — "a matter judged" — is the legal doctrine that once a court of competent jurisdiction has passed a final judgment on a matter, the same matter cannot be relitigated between the same parties. Codified in Section 11 of the Code of Civil Procedure, the doctrine serves the twin purposes of finality (disputes must end at some point) and efficiency (the court system cannot indefinitely revisit the same controversy).
The classical requirements for res judicata: (1) the matter directly and substantially in issue in the later suit was the same as in the earlier; (2) the earlier suit was between the same parties or parties under whom they claim; (3) the earlier suit was litigated in a court competent to try the later suit; (4) the earlier matter was heard and finally decided.
Related doctrines that Indian practice often treats alongside res judicata: *constructive res judicata* (points that ought to have been raised in the earlier suit cannot be raised later, even if not actually raised — Explanation IV to Section 11); *issue estoppel* (specific issues conclusively decided bind the parties in later proceedings even if the causes of action differ).
In criminal practice, an analogous principle — *autrefois convict* and *autrefois acquit* — prevents double jeopardy (reflected in Article 20(2) of the Constitution and Section 337 BNSS / Section 300 CrPC).
Res judicata is typically raised at the earliest opportunity — in the written statement for civil suits, or as a preliminary objection in appellate and writ proceedings. A well-pleaded res judicata defence can end a matter at the threshold.