Glossary
Collateral estoppel
A doctrine that prevents relitigation of a specific factual or legal issue that was decided in a prior case. Sometimes called issue preclusion. Different from res judicata, which bars whole claims.
Collateral estoppel is narrower than res judicata. Even if you can bring a new claim that wasn't barred by res judicata, you might still be stuck with the prior court's ruling on a specific fact or issue: say, that you breached a particular contract or that a particular event happened.
The doctrine requires that the issue was actually litigated and necessary to the prior judgment. It also requires (in some formulations) that the party against whom estoppel is asserted had a full and fair opportunity to litigate the issue the first time.
Collateral estoppel can be powerful in serial litigation: losing a key fact in one case can lock that loss in for every related case afterward.