The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

stimulus
    n 1: any stimulating information or event; acts to arouse action
         [syn: stimulation, stimulus, stimulant, input]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "stimulus":
   activator, aggravation, agitation, animation, animator,
   antecedents, arousal, arouser, arousing, bait, base, basis, boost,
   bribe, call, carrot, catalyst, causation, cause, cause and effect,
   determinant, determinative, electrification, element,
   encouragement, energizer, etiology, exacerbation, exasperation,
   excitation, excitement, exhilaration, factor, fillip, fomentation,
   galvanization, goad, ground, grounds, human dynamo, impetus,
   impulse, incentive, incitation, incitement, inducement,
   inflammation, infuriation, instigation, interest, invitation,
   irritation, lathering up, life, lure, motivating force, motivation,
   motive, motive power, occasion, payment, percentage, persuasive,
   perturbation, principle, profit, propellant, provocation,
   provocative, push, restorative, reward, spark plug, spur,
   steaming up, stimulant, stimulation, stimulative, stimulator,
   stirring, stirring up, sweetener, sweetening, tonic, urging, whet,
   whipping up, working up