A federal court's decision not to exercise jurisdiction over a case. The usual goal of abstention is the avoidance of needless conflict with a state court. See Federalism.
Abstention: an overview
Abstention is a doctrine under which federal courts may choose not to hear a case, even if all the formal jurisdiction requirements are met. There are several established instances in which federal courts will generally abstain. First, federal courts will abstain rather than issue an injunction against a state court, in either a civil or criminal matter. See Klein v Burke Constr. Co, 260 US 266 (1922); Younger v. Harris, 401 U.S. 37 (1971). Second, a federal court can abstain if the case presents unresolved questions of both the state law and the federal constitution. In that case, the federal court generally would want to avoid the constitutional issue if possible, but also does not want to get the state law question wrong. Therefore, in a practice called “Pullman abstention,” the federal court may abstain until the state law question can be resolved in state court. See Railroad Comm'n of Texas v. Pullman Co., 312 US 496 (1941).