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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "tropism":
   a thing for, affinity, aptitude, aptness, bent, bias, cast,
   conatus, conduciveness, delight, diathesis, disposition, eagerness,
   feeling for, inclination, leaning, liability, liking, penchant,
   predilection, predisposition, prejudice, probability, proclivity,
   proneness, propensity, readiness, sensitivity to, soft spot,
   susceptibility, tendency, turn, twist, warp, weakness,
   willingness