Doctrine allowing appeals from interlocutory rulings (i.e., preceding final judgment) so long as those rulings conclusively decide an issue separate from the merits of the case and would be effectively unreviewable after final judgment.
The collateral order doctrine is a narrow exception to the final-judgment rule, which normally forces parties to wait for final judgment before appealing any rulings. This doctrine emerged in Cohen v. Beneficial Industrial Loan Corp., 337 U.S. 541 (1949). In that case the Supreme Court held to be appealable those orders which "finally determine claims of right separable from, and collateral to, rights asserted in the action, too important to be denied review and too independent of the cause itself to require that appellate consideration be deferred until the whole case is adjudicated." Id., 337 U.S. at 546.