The Collaborative International Dictionary of English v.0.48:

Collude \Col*lude"\, v. i. [imp. & p. p. Colluded; p. pr. &
   vb. n. Colluding.] [L. colludere, -lusum; col- + ludere to
   play. See Ludicrous.]
   To have secretly a joint part or share in an action; to play
   into each other's hands; to conspire; to act in concert.
   [1913 Webster]

         If they let things take their course, they will be
         represented as colluding with sedition.  --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

collude
    v 1: act in unison or agreement and in secret towards a
         deceitful or illegal purpose; "The two companies conspired
         to cause the value of the stock to fall" [syn: conspire,
         collude]