The Collaborative International Dictionary of English v.0.48:

Spiritualism \Spir"it*u*al*ism\, n.
   1. The quality or state of being spiritual.
      [1913 Webster]

   2. (Physiol.) The doctrine, in opposition to the
      materialists, that all which exists is spirit, or soul --
      that what is called the external world is either a
      succession of notions impressed on the mind by the Deity,
      as maintained by Berkeley, or else the mere educt of the
      mind itself, as taught by Fichte.
      [1913 Webster]

   3. A belief that departed spirits hold intercourse with
      mortals by means of physical phenomena, as by rapping, or
      during abnormal mental states, as in trances, or the like,
      commonly manifested through a person of special
      susceptibility, called a medium; spiritism; the doctrines
      and practices of spiritualists.
      [1913 Webster]

            What is called spiritualism should, I think, be
            called a mental species of materialism. --R. H.
                                                  Hutton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spiritualism
    n 1: (theology) any doctrine that asserts the separate existence
         of God
    2: the belief that the spirits of dead people can communicate
       with people who are still alive (especially via a medium)
    3: concern with things of the spirit [syn: spirituality,
       spiritualism, spiritism, otherworldliness] [ant:
       worldliness]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "spiritualism":
   Berkeleianism, Hegelianism, Kantianism, Neoplatonism,
   Platonic form, Platonic idea, Platonism, absolute idealism,
   animatism, animism, form, hylozoism, idealism, immaterialism,
   mediumism, metaphysical idealism, monistic idealism, necromancy,
   panpsychism, personalism, psychism, seance, sitting, solipsism,
   spirit, spiritism, subjectivism, transcendental, universal