The Collaborative International Dictionary of English v.0.48:

mentalism \mentalism\ n.
   1. a doctrine that mind is the only true reality and that
      objects exist only as aspects of the mind's awareness.
      [WordNet 1.5]

   2. Any theory of psychology that accepts introspective data
      about the functioning of the mind as a valid object of
      study; contrasted to behaviorism.
      [PJC]

WordNet (r) 3.0 (2006):

mentalism
    n 1: (philosophy) a doctrine that mind is the true reality and
         that objects exist only as aspects of the mind's awareness