Collateral Estoppel

Collateral Estoppel. A doctrine, also called issue preclusion, that prevents a party from relitigating a specific factual or legal issue that was actually decided against it in a prior case, even if the new lawsuit involves a different claim. The issue must have been actually litigated, necessarily decided, and essential to the earlier judgment. Unlike res judicata, it targets individual issues rather than whole claims. Example: Having lost on patent invalidity in the first suit, the manufacturer was collaterally estopped from arguing invalidity against a different accuser.