The Collaborative International Dictionary of English v.0.48:

Incentive \In*cen"tive\, n. [L. incentivum.]
   That which moves or influences the mind, or operates on the
   passions; that which incites, or has a tendency to incite, to
   determination or action; that which prompts to good or ill;
   motive; spur; as, the love of money, and the desire of
   promotion, are two powerful incentives to action.
   [1913 Webster]

         The greatest obstacles, the greatest terrors that come
         in their way, are so far from making them quit the work
         they had begun, that they rather prove incentives to
         them to go on in it.                     --South.

   Syn: Motive; spur; stimulus; incitement; encouragement;
        inducement; influence.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Incentive \In*cen"tive\, a. [L. incentivus, from incinere to
   strike up or set the tune; pref. in- + canere to sing. See
   Enchant, Chant.]
   [1913 Webster]
   1. Inciting; encouraging or moving; rousing to action;
      stimulative.
      [1913 Webster]

            Competency is the most incentive to industry. --Dr.
                                                  H. More.
      [1913 Webster]

   2. Serving to kindle or set on fire. [R.]
      [1913 Webster]

            Part incentive reed
            Provide, pernicious with one touch of fire.
      --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incentive
    n 1: a positive motivational influence [syn: incentive,
         inducement, motivator] [ant: deterrence,
         disincentive]
    2: an additional payment (or other remuneration) to employees as
       a means of increasing output [syn: bonus, incentive]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "incentive":
   agitational, agitative, bait, bribe, carrot, catalyst, clout,
   compulsion, drive, driving force, encouragement, enticement,
   fillip, fomenting, goad, impellent, impelling force, impetus,
   impulse, impulsion, incendiary, incitement, inciting, incitive,
   inducement, inflammatory, instigating, instigative, interest,
   invitation, irresistible force, lure, moment, momentum, motivation,
   motive power, payment, percentage, persuasive, power, prod, profit,
   propulsion, provocation, rabble-rousing, reward, spur, stimulant,
   stimulation, stimulative, stimulus, sweetener, sweetening, thrust,
   whet