A sub-category of Federal Subject Matter Jurisdiction :
"The district courts shall have original jurisdiction of all civil actions arising under the Constitution, laws, or treaties of the United States." (28 U.S. Code § 1331 - Federal question)
A federal question exists if the cause of action is expressly created by or can be fairly implied by federal law. Otherwise, the complaint must involve a real and substantial issue of federal law and its determination must necessarily depend on resolution of the case.