Latin: A matter which has already been conclusively decided by a court between the parties.
Final judgments prevent any re-examination or re-trial of the same dispute between the same parties.
And as noted in the Henderson v. Henderson (1843) 3 Hare 100 (a British law report):
"In trying this question, I believe I state the rule of the court correctly, when I say, that where a given matter becomes the subject of litigation in, and of adjudication by, a court of competent jurisdiction, the court requires the parties to that litigation to bring forward their whole case, and will not (except under special circumstances) permit the same parties to open the same subject of litigation in respect of matter which might have been brought forward as part of the subject in contest, but which was not brought forward, only because they have, from negligence, inadvertence, or even accident, omitted part of their case. The plea of res judicata applies, except in special cases, not only to points upon which the court was actually required by the parties to form an opinion and pronounce a judgment, but to every point which properly belonged to the subject of litigation, and which the parties, exercising reasonable diligence, might have brought forward at the time."
They are not, though, conclusive in respect of disputes on the same subject between different parties or between the same parties on a different subject matter.