The Collaborative International Dictionary of English v.0.48:

Militant \Mil"i*tant\, a. [L. militans, -antis, p. pr. of
   militare to be soldier: cf. F. militant. See Militate.]
   Engaged in warfare; fighting; combating; serving as a
   soldier. -- Mil"i*tant*ly, adv.
   [1913 Webster]

         At which command the powers militant . . .
         Moved on in silence.                     --Milton.
   [1913 Webster]

   Church militant, the Christian church on earth, which is
      supposed to be engaged in a constant warfare against its
      enemies, and is thus distinguished from the church
      triumphant, in heaven.
      [1913 Webster]

WordNet (r) 3.0 (2006):

militant
    adj 1: disposed to warfare or hard-line policies; "militant
           nations"; "hawkish congressman"; "warlike policies" [syn:
           militant, hawkish, warlike]
    2: showing a fighting disposition; "highly competitive sales
       representative"; "militant in fighting for better wages for
       workers"; "his self-assertive and ubiquitous energy" [syn:
       competitive, militant]
    3: engaged in war; "belligerent (or warring) nations" [syn:
       belligerent, militant, war-ridden, warring]
    n 1: a militant reformer [syn: militant, activist]