What is Ancillary Jurisdiction?

Legal Definition
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.
Legal Definition
the right of a court that can aid and regulate decisions reached in an original suit but in a different court.