The Collaborative International Dictionary of English v.0.48:

Instigate \In"sti*gate\, v. t. [imp. & p. p. Instigated; p.
   pr. & vb. n. Instigating.] [L. instigatus, p. p. of
   instigare to instigate; pref. in- in + a root akin to G.
   stechen to prick, E. stick. See Stick.]
   To goad or urge forward; to set on; to provoke; to incite; --
   used chiefly with reference to evil actions; as, to instigate
   one to a crime.
   [1913 Webster]

         He hath only instigated his blackest agents to the very
         extent of their malignity.               --Bp.
                                                  Warburton.

   Syn: To stimulate; urge; spur; provoke; tempt; incite; impel;
        encourage; animate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instigate
    v 1: provoke or stir up; "incite a riot"; "set off great unrest
         among the people" [syn: incite, instigate, set off,
         stir up]
    2: serve as the inciting cause of; "She prompted me to call my
       relatives" [syn: prompt, inspire, instigate]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "instigate":
   abet, activate, actuate, agitate, arouse, blow the coals, bring,
   bring on, call forth, contrive, draw down, draw on, effect, elicit,
   evoke, excite, fan, fan the flame, feed the fire, ferment, fire,
   foment, get, goad, heat, heat up, hint, impassion, incense, incite,
   induce, inflame, insinuate, inspire, lead, motivate, nettle,
   obtain, pique, plan, plot, procure, provoke, put up to, raise,
   rally, scheme, set, set on, sic on, stir the embers, stir up,
   suggest, superinduce, tickle, urge, whet, whip up, work up