The Collaborative International Dictionary of English v.0.48:

Stimulant \Stim"u*lant\, a. [L. stimulans, p. pr.; cf. F.
   stimulant. See Stimulate.]
   1. Serving to stimulate.
      [1913 Webster]

   2. (Physiol.) Produced increased vital action in the
      organism, or in any of its parts.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Stimulant \Stim"u*lant\, n. [Cf. F. stimulant.]
   1. That which stimulates, provokes, or excites.
      [1913 Webster]

            His feelings had been exasperated by the constant
            application of stimulants.            --Macaulay.
      [1913 Webster]

   2. (Physiol. & Med.) An agent which produces a temporary
      increase of vital activity in the organism, or in any of
      its parts; -- sometimes used without qualification to
      signify an alcoholic beverage used as a stimulant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stimulant
    adj 1: that stimulates; "stimulant phenomena" [syn: stimulant,
           stimulating]
    n 1: any stimulating information or event; acts to arouse action
         [syn: stimulation, stimulus, stimulant, input]
    2: a drug that temporarily quickens some vital process [syn:
       stimulant, stimulant drug, excitant]