Active militia refers to a volunteer defense unit other than the National Guard.
The following is an example of a case law on active militia:
An act to provide for the organization of the State militia, which exempts an active member of a company of the State militia from serving upon juries, is a valid and constitutional law. The organization of the active militia of the State is not in violation of that clause of the Federal constitution which withholds from the States the right to keep troops in time of peace. Such a militia is not embraced in the term troops, as used in the constitution. The State militia is simply a domestic force, as distinguished from regular troops, and is only liable to be called into service when the exigencies of the State make it necessary. [Dunne v. People, 94 Ill. 120 (Ill. 1879)].