The Collaborative International Dictionary of English v.0.48:

Collusion \Col*lu"sion\, n. [L. collusio: cf. F. collusion. See
   Collude.]
   1. A secret agreement and cooperation for a fraudulent or
      deceitful purpose; a playing into each other's hands;
      deceit; fraud; cunning.
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            The foxe, maister of collusion.       --Spenser.
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            That they [miracles] be done publicly, in the face
            of the world, that there may be no room to suspect
            artifice and collusion.               --Atterbury.
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            By the ignorance of the merchants or dishonesty of
            the weavers, or the collusion of both, the ware was
            bad and the price excessive.          --Swift.
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   2. (Law) An agreement between two or more persons to defraud
      a person of his rights, by the forms of law, or to obtain
      an object forbidden by law. --Bouvier. Abbott.

   Syn: Collusion, Connivance.

   Usage: A person who is guilty of connivance intentionally
          overlooks, and thus sanctions what he was bound to
          prevent. A person who is guilty of collusion unites
          with others (playing into their hands) for fraudulent
          purposes.
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WordNet (r) 3.0 (2006):

collusion
    n 1: secret agreement
    2: agreement on a secret plot [syn: connivance, collusion]