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]