'Tropism' definitions:

Definition of 'tropism'

From: WordNet
noun
An involuntary orienting response; positive or negative reaction to a stimulus source

Definition of 'Tropism'

From: GCIDE
  • 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]