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]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "psychology":
   affect, affectivity, anatomy, anthropography, anthropologist,
   anthropology, anthropometry, attitude, behavior,
   behavioral science, constitution, craniology, craniometry,
   demography, emotion, emotions, emotivity, ethnography, ethnologist,
   ethnology, feeling, feeling tone, feelings, human ecology,
   human geography, mental attitude, nature, opinion, position,
   posture, psyche, rationale, reasoning, science of man, sentiment,
   sociologist, sociology, stance, thinking, way of thinking