In the United States, the organized militia forces of the individual states, that is is trained, and has its officers appointed, by the States according to the discipline prescribed by Congress*; is organized, armed, and equipped wholly or partly at Federal expense; and is federally recognized. It is a component of the Army of the United States when called into active federal service. The National Guard consists of the Army National Guard and the Air National Guard.
* - U.S. Constitution, article I, section 8, clause sixteen
See:
U.S. Code title 32 § 101