The Collaborative International Dictionary of English v.0.48:

Mysticism \Mys"ti*cism\, n. [Cf. F. mysticisme.]
   1. Obscurity of doctrine.
      [1913 Webster]

   2. (Eccl. Hist.) The doctrine of the Mystics, who professed a
      pure, sublime, and wholly disinterested devotion, and
      maintained that they had direct intercourse with the
      divine Spirit, and aquired a knowledge of God and of
      spiritual things unattainable by the natural intellect,
      and such as can not be analyzed or explained.
      [1913 Webster]

   3. (Philos.) The doctrine that the ultimate elements or
      principles of knowledge or belief are gained by an act or
      process akin to feeling or faith.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mysticism
    n 1: a religion based on mystical communion with an ultimate
         reality [syn: mysticism, religious mysticism]
    2: obscure or irrational thought

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "mysticism":
   afflatus, anagoge, anagogics, anthroposophy, apocalypse, cabala,
   cabalism, direct communication, divine inspiration,
   divine revelation, epiphany, esotericism, esoterics, esoterism,
   esotery, hocus-pocus, inspiration, mumbo jumbo, mystery,
   mystical experience, mystification, occultism, prophecy,
   revelation, symbolics, symbolism, theophania, theophany,
   theopneustia, theopneusty, yoga, yogeeism, yogism