The Collaborative International Dictionary of English v.0.48:

Conspirator \Con*spir"a*tor\, n.
   One who engages in a conspiracy; a plotter. --2 Sam. xv. 31.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conspirator
    n 1: a member of a conspiracy [syn: conspirator,
         coconspirator, plotter, machinator]