Ancillary Jurisdiction Definition

A term that describes a federal court's authority to hear certain legal claims that normally aren't under federal jurisdiction, when those claims arise out of the same transaction or occurrence giving rise to a federal claim.

For example, suppose a federal court lawsuit claims the government violated equal protection guarantees under the U.S. Constitution and a state constitution. The federal court has federal question jurisdiction over the claim that the government's action violates the U.S. Constitution. But the state constitutional claim normally isn't under federal court jurisdiction. In the interest of efficiency, the court is likely to assert ancillary jurisdiction over the state constitution claim so it can resolve all issues in the case.

(See also: pendent jurisdiction)