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The Collaborative International Dictionary of English v.0.48 (gcide)

Stimulus \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.
[1913 Webster]

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.
[1913 Webster]

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.
[1913 Webster]