Ancillary jurisdiction allows a federal court to hear a claim that would normally be outside of its subject-matter jurisdiction if it is substantially related to a second claim that is within the court's jurisdiction. A claim comes within a federal court’s ancillary jurisdiction when it bears a logical relationship to the aggregate core of operative facts of the main claim, and the main claim meets the requirements of federal question jurisdiction, or diversity jurisdiction.
See: e.g., Hendrickson v. U.S 791 F.3d 354 (2015)
[Last updated in June of 2022 by the Wex Definitions Team]