Source : Webster's Revised Unabridged Dictionary (1913)

Excitant \Ex*cit"ant\, a. [L. excitans, -antis, p. pr. of
   excitare: cf. F. excitant.]
   Tending to excite; exciting.

Excitant \Ex*cit"ant\, n. (Physiol.)
   An agent or influence which arouses vital activity, or
   produces increased action, in a living organism or in any of
   its tissues or parts; a stimulant.

Source : WordNet®

excitant
     adj : (of drugs e.g.) able to excite or stimulate [syn: {excitative},
            {excitatory}]