Collateral estoppel is a legal doctrine which allows people to block relitigation of an issue which has already been decided in a court of law. The person against whom this doctrine is used is said to be estopped. The goal of collateral estoppel is to reduce the load on the legal system by weeding out repeat or nuisance cases and to prevent harassment by allowing people to estop parties who attempt to relitigate decided issues.
This doctrine arose in civil law, although it is sometimes also applied to criminal law. Essentially, when a party is estopped under collateral estoppel, it is because it has been successfully argued that the matter was already decided and a firm conclusion was reached. If the legal decision in the previous litigation was valid, another litigation cannot proceed.
Defendants can utilize collateral estoppel to prevent relitigation of a case they've already been involved in, under the argument that the issue has been satisfactorily resolved and they should not be subject to additional litigation. People can also use this doctrine when they were not directly involved in the initial litigation, arguing that the first suit decided the issue and it cannot be tried again. While these tactics are used by defendants, plaintiffs can also call upon collateral estoppel to argue that a case has been decided and litigation should not be reopened.
People may use the term “issue preclusion” to describe collateral estoppel. This references the fact that the doctrine rests on the idea that once an issue has been decided conclusively, it cannot be relitigated. However, collateral estoppel does not bar appeals; people can argue that the decision was invalid or problematic in some way, and that as a result they should have the right to appeal so that they can receive a new trial.
Some concerns have been raised about the impact that this doctrine can have on due process. Many nations recognize that people do have a legal right to take issues to court, and anything which restricts this right can be problematic. Some scholars have pointed out that collateral estoppel could be used in a way which abridges rights, especially those of defendants. This must be weighed when considering cases in which people wish to use the doctrine, to confirm that issue preclusion is indeed warranted in that case and that no one's rights will be restricted if the doctrine is utilized.