The Collaborative International Dictionary of English v.0.48:

Excitant \Ex*cit"ant\, a. [L. excitans, -antis, p. pr. of
   excitare: cf. F. excitant.]
   Tending to excite; exciting.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

excitant
    adj 1: (of drugs e.g.) able to excite or stimulate [syn:
           excitant, excitative, excitatory]
    n 1: a drug that temporarily quickens some vital process [syn:
         stimulant, stimulant drug, excitant]