Civil Procedure
Collateral Estoppel (Issue Preclusion)
Definition
Collateral estoppel, or issue preclusion, prevents the relitigation of specific factual or legal issues that were actually litigated and necessarily decided in a prior proceeding. Unlike res judicata, which bars entire claims, collateral estoppel applies to discrete issues. Traditionally, mutuality of parties was required, but the modern trend allows non-mutual offensive and defensive use of collateral estoppel, subject to fairness considerations.
Example
A court finds that a defendant ran a red light. In a subsequent suit by a different plaintiff, the defendant is collaterally estopped from relitigating whether they ran the red light.