The Collaborative International Dictionary of English v.0.48:

Psychophysics \Psy`cho*phys"ics\, n. [Psycho- + physics.]
   The science of the connection between nerve action and
   consciousness; the science which treats of the relations of
   the psychical and physical in their conjoint operation in
   man; the doctrine of the relation of function or dependence
   between body and soul.
   [1913 Webster]

WordNet (r) 3.0 (2006):

psychophysics
    n 1: the branch of psychology concerned with quantitative
         relations between physical stimuli and their psychological
         effects