Define "collateral estoppel".

The term "collateral estoppel" refers to a legal doctrine that prohibits the re-litigation of issues that have already been conclusively settled in a prior legal action. This principle is based on the notion of judicial efficiency and fairness; once an issue has been fully and fairly adjudicated, allowing it to be re-litigated would undermine the finality of judgments and could lead to inconsistent results.

In practical terms, if an issue of fact or law has been decided in a previous case involving the same parties (or their privies), that decision is binding in subsequent cases. This significantly reduces court congestion by preventing endless litigation over the same matters, ensuring that parties cannot relitigate issues that have already been resolved.

This legal doctrine ensures that once a party has had a fair opportunity to litigate an issue, they cannot return to court to challenge it again in a different context, thus providing closure and upholding the integrity of the judicial system.