Glossary
Res judicata
The doctrine that prevents a party from suing again on a claim that has already been finally decided in court. Latin for "a thing decided." Sometimes called claim preclusion.
Res judicata bars relitigation of the same claim. Once a court has issued a final judgment on the merits, the same parties can't try again: they can appeal, but they can't just refile.
The doctrine has three main requirements: same parties (or those in privity with them), same claim, and a prior final judgment on the merits. If all three are met, the new lawsuit is barred.
Res judicata is broader than it sounds. It bars not just the exact same legal theory, but any claim that arose out of the same facts and could have been brought in the prior lawsuit. That's why complaints often plead every possible theory in one lawsuit: splitting them risks losing everything you didn't include.