Roughly translates as “a matter already judged” or previously decided.
This is a Latin saying that expresses the principle that where a matter of fact or point of law has already been decided, there is no justification for re-litigating or deciding it afresh. The principle is intended to promote certainty and the efficient use of judicial resources, as well as protecting litigants from abuse of process.
The fact that a matter is res judicata does not preclude an appeal.
See also: estoppel.