'Instigator' definitions:

Definition of 'instigator'

From: WordNet
noun
Someone who deliberately foments trouble; "she was the instigator of their quarrel" [syn: instigator, provoker, inciter, instigant, firebrand]
noun
A person who initiates a course of action [syn: instigator, initiator]

Definition of 'Instigator'

From: GCIDE
  • Instigator \In"sti*ga`tor\, n. [L.: cf. F. instigateur.] One who instigates or incites. --Burke.
  • Syn: inciter, instigant. [1913 Webster]