\Stim"u*lus\, n.; pl. {Stimuli}. [L., for stigmulus,
akin to L. instigare to stimulate. See {Instigare}, {Stick},
v. t.]
1. A goad; hence, something that rouses the mind or spirits;
   an incentive; as, the hope of gain is a powerful stimulus
   to labor and action.

2. That which excites or produces a temporary increase of
   vital action, either in the whole organism or in any of
   its parts; especially (Physiol.), any substance or agent
   capable of evoking the activity of a nerve or irritable
   muscle, or capable of producing an impression upon a
   sensory organ or more particularly upon its specific end
   organ.

Note: Of the stimuli applied to the sensory apparatus,
      physiologists distinguish two kinds: (a) {Homologous
      stimuli}, which act only upon the end organ, and for
      whose action the sense organs are especially adapted,
      as the rods and cones of the retina for the vibrations
      of the either. (b) {Heterologous stimuli}, which are
      mechanical, chemical, electrical, etc., and act upon
      the nervous elements of the sensory apparatus along
      their entire course, producing, for example, the flash
      of light beheld when the eye is struck. --Landois &
      Stirling.