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.
      [1913 Webster]

            The foxe, maister of collusion.       --Spenser.
      [1913 Webster]

            That they [miracles] be done publicly, in the face
            of the world, that there may be no room to suspect
            artifice and collusion.               --Atterbury.
      [1913 Webster]

            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.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "collusion":
   accompaniment, accordance, agreement, alliance, artifice,
   association, bipartisanship, cabal, cahoots, chicane, chicanery,
   co-working, coaction, coadjuvancy, coadministration, coagency,
   cochairmanship, codirectorship, coincidence, collaboration,
   collaborativeness, collectivism, collectivity, combination,
   combined effort, commensalism, common effort, common enterprise,
   communalism, communism, communitarianism, community, complicity,
   complot, concert, concerted action, concomitance, concord,
   concordance, concourse, concurrence, confederacy, confluence,
   conjunction, connivance, connivery, consilience, conspiracy,
   contrivance, contriving, cooperation, cooperativeness,
   correspondence, counterplot, covin, deep-laid plot, dodgery, duet,
   duumvirate, ecumenicalism, ecumenicism, ecumenism, engineering,
   esprit, esprit de corps, fellow feeling, fellowship, finagling,
   finesse, foul play, frame-up, game, harmony, intrigue,
   joining of forces, joint effort, joint operation, junction,
   little game, machination, maneuvering, manipulation, mass action,
   morale, mutual assistance, mutualism, mutuality, octet, parasitism,
   pettifoggery, pettifogging, plot, plotting, pooling,
   pooling of resources, pulling together, quartet, quintet,
   reciprocity, rigging, saprophytism, scheme, schemery, scheming,
   septet, sextet, sharp practice, simultaneity, skulduggery, sleight,
   solidarity, stratagem, supercherie, symbiosis, synchronism,
   synergism, synergy, team spirit, teamwork, trick, trickery, trio,
   triumvirate, troika, underhand dealing, underplot, union,
   united action, web of intrigue, wire-pulling



Bouvier's Law Dictionary, Revised 6th Ed (1856):

COLLUSION, fraud. 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; as, for example, where the husband and wife collude to obtain a
divorce for a cause not authorized by law. It is nearly synonymous with
@covin. (q.v.)
     2. Collusion and fraud of every kind vitiate all acts which are
infected with them, and render them void. Vide Shelf. on Mar. & Div. 416,
450; 3 Hagg. Eccl. R. 130, 133; 2 Greenl. Ev. Sec. 51; Bousq. Dict. de Dr.
mot Abordage.