A natural susceptibility, characteristic of all
   living organisms, tissues, and cells, to the influence of certain
   stimuli, response being manifested in a variety of ways, -- as that
   quality in plants by which they exhibit motion under suitable
   stimulation; esp., the property which living muscle processes, of
   responding either to a direct stimulus of its substance, or to the
   stimulating influence of its nerve fibers, the response being indicated
   by a change of form, or contraction; contractility.