Definition of Younger doctrine
The principle, developed by the Supreme Court in Younger v. Harris, 401 U.S. 37, 91 S.Ct. 746, 27 L.Ed.2d 669 and other cases, that federal courts should not interfere with an ongoing state criminal proceeding, either by injunction or declaratory relief, unless the prosecution has been brought in bad faith or as harassment. The Younger doctrine has also been applied in civil actions where the state is a party and important state substantive goals are implicated.
That's the definition of Younger doctrine in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.