The term militia in the United States has been defined and modified by Congress several times throughout U.S. history. As a result, the meaning of "the militia" is complex and has transformed over time. It has historically been used to describe all able-bodied men who are not members of the Army or Navy (Uniformed Services). From the U.S. Constitution, Article II (The Executive branch), Sec. 2, Clause 1: "The President shall be the Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States when called into the actual service of the United States."
Today, the term militia is used to describe a number of groups within the United States. Primarily, these are:
The organized militia defined by the Militia Act of 1903, which repealed section two hundred thirty-two and sections 1625 - 1660 of title sixteen of the Revised Statutes, consists of State militia forces, notably the National Guard and the Naval Militia. The National Guard, however, is not to be confused with the National Guard of the United States, which is a federally recognized reserve military force, although the two are linked.
The reserve militia are part of the unorganized militia defined by the Militia Act of 1903 as consisting of every able-bodied man of at least 17 and under 45 years of age who is not a member of the National Guard or Naval Militia.
Former members of the armed forces are also considered part of the "unorganized militia" per Sec 313 Title 32 of the US Code.