The Collaborative International Dictionary of English v.0.48:

activism \activism\ n.
   1. a policy of taking direct and militant action to achieve a
      political or social goal.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

activism
    n 1: a policy of taking direct and militant action to achieve a
         political or social goal

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "activism":
   act, acting, action, activeness, activity, behavior, business,
   doing, doings, employment, exercise, function, functioning,
   goings-on, militancy, motion, movement, movements, occupation,
   operation, operations, play, political activism, practice, praxis,
   proceedings, stir, swing, work, working, workings