What is Collateral Estoppel?

Legal Definition
Collateral estoppel (CE), known in modern terminology as issue preclusion, is a common law estoppel doctrine that prevents a person from relitigating an issue. One summary is that, "once a court has decided an issue of fact or law necessary to its judgment, that decision ... preclude[s] relitigation of the issue in a suit on a different cause of action involving a party to the first case". The rationale behind issue preclusion is the prevention of legal harassment and the prevention of abuse of judicial resources.
-- Wikipedia
Legal Definition
The doctrine of collateral estoppel, a common law legacy codified by Ashe v. Swenson 397 U.S. 436 (1970), protects criminal defendants from being tried for the same issue in more than one criminal trial. In Ashe v. Swenson, the Court ruled that the aegis of the Fifth Amendment's protections against double jeopardy are enforceable in state as well as federal court through the Due Process Clause of the Fourteenth Amendment as established by Benton v. Maryland 395 U.S. 784. This decision relies on the application of the Full Faith and Credit Clause of the Constitution.

As a subgenre of res judicata, collateral estoppel prevents subsequent litigation of legal determinations of fact and law that have resulted in valid final judgments. All litigants have a "full and fair" opportunity to bring suit except where one party has brought effectively the same suit as defined by the same substantive legal issue in another venue or at another time against the same defendant.

See also San Remo Hotel v. San Francisco 545 U.S. 323 (2005).
Legal Definition
an estoppel by judgement where an issue that has been decided cannot be taken to another court by the same litigants and in the same manner.
Legal Definition
The collateral determination of a question by a court having general jurisdiction of the subject. See Small v. Haskins, 26 Vt. 209.
-- Black's Law Dictionary