The Collaborative International Dictionary of English v.0.48:

Psychology \Psy*chol"o*gy\, n. pl. Psychologies. [Psycho- +
   -logy: cf. F. psychologie. See Psychical.]
   The science of the human soul; specifically, the systematic
   or scientific knowledge of the powers and functions of the
   human soul, so far as they are known by consciousness; a
   treatise on the human soul.
   [1913 Webster]

         Psychology, the science conversant about the phenomena
         of the mind, or conscious subject, or self. --Sir W.
                                                  Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

psychology
    n 1: the science of mental life [syn: psychology,
         psychological science]