The Collaborative International Dictionary of English v.0.48:

Collaboration \Col*lab`o*ra"tion\, n.
   1. The act of working together; united labor.
      [1913 Webster]

   2. the act of willingly cooperating with an enemy, especially
      an enemy nation occupying one's own country.
      [PJC]

WordNet (r) 3.0 (2006):

collaboration
    n 1: act of working jointly; "they worked either in
         collaboration or independently" [syn: collaboration,
         coaction]
    2: act of cooperating traitorously with an enemy that is
       occupying your country [syn: collaboration,
       collaborationism, quislingism]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "collaboration":
   accompaniment, accordance, agreement, alliance, association,
   bipartisanship, cahoots, co-working, coaction, coadjuvancy,
   coadministration, coagency, cochairmanship, codirectorship,
   coincidence, collaborativeness, collectivism, collectivity,
   collusion, combination, combined effort, commensalism,
   common effort, common enterprise, communalism, communism,
   communitarianism, community, complicity, concert, concerted action,
   concomitance, concord, concordance, concourse, concurrence,
   confluence, conjunction, consilience, conspiracy, cooperation,
   cooperativeness, correspondence, duet, duumvirate, ecumenicalism,
   ecumenicism, ecumenism, esprit, esprit de corps, fellow feeling,
   fellowship, fifth-column activity, fraternization, harmony,
   high treason, joining of forces, joint effort, joint operation,
   junction, lese majesty, mass action, misprision of treason, morale,
   mutual assistance, mutualism, mutuality, octet, parasitism,
   petty treason, pooling, pooling of resources, pulling together,
   quartet, quintet, quislingism, reciprocity, saprophytism, sedition,
   septet, sextet, simultaneity, solidarity, symbiosis, synchronism,
   synergism, synergy, team spirit, teamwork, treason, trio,
   triumvirate, troika, union, united action