The Collaborative International Dictionary of English v.0.48:

Satanism \Sa"tan*ism\, n.
   1. The evil and malicious disposition of Satan; a diabolical
      spirit. [R.]
      [1913 Webster]

   2. The worship of Satan.
      [PJC]

WordNet (r) 3.0 (2006):

Satanism
    n 1: a belief in and reverence for devils (especially Satan)
         [syn: diabolism, demonism, Satanism]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "Satanism":
   Black Mass, black magic, chthonian worship, demonism, demonography,
   demonolatry, demonology, demonomancy, demonomy, demonry,
   devil lore, devil worship, devilry, diablerie, diabolism,
   diabology, diabolology, sorcery, the black art