Federal question jurisdiction refers to the jurisdiction of United States federal courts to hear a civil case when plaintiff has alleged a violation of federal law, the US Constitution, or a treaty which the US is included in. Initially, lower federal courts did not posses this jurisdiction to prevent them from becoming too powerful.
After 1980, federal question jurisdiction has no requirement of jurisdictional amount. Now any federal court can hear such a case even if plaintiff is seeking no money. In order to be heard by a federal court, the plaintiff must include the federal question in his cofmplaint.