The Collaborative International Dictionary of English v.0.48:

Tropism \Tro"pism\ (tr[=o]"p[i^]z'm), n. [Gr. troph` a turning,
   tre`pein to turn + -ism.] (Physiol.)
   Modification of the direction of growth, caused by some
   external influence, such as light; -- sometimes used for
   motion of an organism toward or away from an external
   stimulus, more properly called taxis.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

tropism
    n 1: an involuntary orienting response; positive or negative
         reaction to a stimulus source