The Collaborative International Dictionary of English v.0.48:

Paganism \Pa"gan*ism\ (-[i^]z'm), n. [L. paganismus: cf. F.
   paganisme. See Pagan, and cf. Painim.]
   The state of being pagan; pagan characteristics; esp., the
   worship of idols or false gods, or the system of religious
   opinions and worship maintained by pagans; heathenism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

paganism
    n 1: any of various religions other than Christianity or Judaism
         or Islamism [syn: paganism, pagan religion,
         heathenism]