Source : Webster's Revised Unabridged Dictionary (1913)

Immaterialism \Im`ma*te"ri*al*ism\, n. [Cf. F.
   immat['e]rialisme.]
   1. The doctrine that immaterial substances or spiritual being
      exist, or are possible.

   2. (Philos.) The doctrine that external bodies may be reduced
      to mind and ideas in a mind; any doctrine opposed to
      materialism or phenomenalism, esp. a system that maintains
      the immateriality of the soul; idealism; esp., Bishop
      Berkeley's theory of idealism.