U.S. litigation term. Federal Courts have limited jurisdiction based on either:
(a) diversity of citizenship; or
(b) Federal Questions, i.e., cases arising under Federal law such as Copyright, Patent, Federal Trademark and Federal Antitrust (and others).
Where a state law claim is based on a “common nucleus of operative fact,” i.e., the same facts or events are at issue, Federal Courts can hear the claims based on state law, if they are simultaneously hearing a case based on Federal Question jurisdiction. While jurisdiction arising under (a) diversity is more accurately referred to as Ancillary Jurisdiction, both are generally collectively referred to as “Supplemental Jurisdiction.”