Ancillary jurisdiction allows a United States federal court to hear certain claims sufficiently related to the original claim that would otherwise defeat the court's jurisdiction. Whereas pendent jurisdiction

Pendent jurisdiction

Pendent jurisdiction is the authority of a United States federal court to hear a closely related state law claim against a party already facing a federal claim, described by the Supreme Court as "jurisdiction over nonfederal claims between parties litigating other matters properly before the...

 allows a federal court to hear state claims sufficiently related to an original federal claim, ancillary jurisdiction applies when the parties are in federal court because of diversity

Diversity jurisdiction

In the law of the United States, diversity jurisdiction is a form of subject-matter jurisdiction in civil procedure in which a United States district court has the power to hear a civil case where the persons that are parties are "diverse" in citizenship, which generally indicates that they are...

 (i.e., each defendant is from a state different than each plaintiff) and one party wants to bring a claim against another party (possibly a third-party) which would otherwise defeat that diversity.