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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "paganism":
   Druidism, Gothicism, Parsiism, Sabaism, Zoroastrianism,
   age of ignorance, allotheism, animal worship, animatism, animism,
   barbarism, benightedness, benightment, dark, dark age, darkness,
   demonism, demonolatry, devil worship, fetishism, fire worship,
   heathendom, heathenism, heathenry, hero worship, iconoduly,
   iconolatry, idol worship, idolatrousness, idolatry, idolism,
   idolodulia, idolomancy, image worship, pagandom, paganry,
   phallic worship, phallicism, pyrolatry, savagery, snake worship,
   star worship, sun worship, tree worship, unenlightenment