noun

1. 1. Archaic any military force
   2. later, any army composed of citizens rather than professional soldiers, called up in time of emergency
2. ☆ in the U.S., all able-bodied male citizens between 18 and 45 years old who are not already members of the regular armed forces: members of the National Guard and of the Reserves (of the Army, Air Force, Coast Guard, Navy, and Marine Corps) constitute the organized militia; all others, the unorganized militia
3. any of various disaffected groups of citizens that are organized as to resemble an army and that oppose the authority of the federal government

Etymology: L, military service, soldiery < miles (gen. militis), soldier