The Collaborative International Dictionary of English v.0.48:

Militia \Mi*li"tia\, n. [L., military service, soldiery, fr.
   miles, militis, soldier: cf. F. milice.]
   [1913 Webster]
   1. In the widest sense, the whole military force of a nation,
      including both those engaged in military service as a
      business, and those competent and available for such
      service; specifically, the body of citizens enrolled for
      military instruction and discipline, but not subject to be
      called into actual service except in emergencies.
      [1913 Webster]

            The king's captains and soldiers fight his battles,
            and yet . . . the power of the militia is he. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. Military service; warfare. [Obs.] --Baxter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

militia
    n 1: civilians trained as soldiers but not part of the regular
         army [syn: militia, reserves]
    2: the entire body of physically fit civilians eligible by law
       for military service; "their troops were untrained militia";
       "Congress shall have power to provide for calling forth the
       militia"--United States Constitution