The Collaborative International Dictionary of English v.0.48:

Cooperate \Co*["o]p"er*ate\, v. i. [imp. & p. p. Cooperated;
   p. pr. & vb. n. Cooperating.] [L. cooperatus, p. p. of
   cooperari to cooperate; co + operari to work, opus work. See
   Operate.]
   To act or operate jointly with another or others; to concur
   in action, effort, or effect.
   [1913 Webster]

         Whate'er cooperates to the common mirth. --Crashaw.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cooperate
    v 1: work together on a common enterprise of project; "The
         soprano and the pianist did not get together very well";
         "We joined forces with another research group" [syn:
         collaborate, join forces, cooperate, get together]