(Latin: “a thing adjudged”), a thing or matter that has been finally juridically decided on its merits and cannot be litigated again between the same parties. The term is often used in reference to the maxim that repeated reexamination of adjudicated disputes is not in any society's interest.

It has long been held that one judicial contest is enough for the litigants on a particular claim or defense. As the volume of judicial work has risen, the need to limit litigants to a single contest about a single controversy has become more urgent. The concept of res judicata has expanded in scope and power as the courts have refined its operation.