1. 
[syn: collaborator, cooperator, partner, pardner]
The Collaborative International Dictionary of English v.0.48:
Cooperator \Co*["o]p"er*a`tor\, n. [L.: cf. F. coop['e]rateur.]
   One who labors jointly with others to promote the same end.
   "Cooperators with the truth." --Boyle.
   [1913 Webster]
WordNet (r) 3.0 (2006):
cooperator
    n 1: an associate in an activity or endeavor or sphere of common
         interest; "the musician and the librettist were
         collaborators"; "sexual partners" [syn: collaborator,
         cooperator, partner, pardner]