The Collaborative International Dictionary of English v.0.48:

Cooperation \Co*["o]p`er*a"tion\, n. [L. cooperatio: cf. F.
   coop['e]ration.]
   1. The act of cooperating, or of operating together to one
      end; joint operation; concurrent effort or labor.
      [1913 Webster]

            Not holpen by the cooperation of angels. --Bacon.
      [1913 Webster]

   2. (Polit. Econ.) The association of a number of persons for
      their benefit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cooperation
    n 1: joint operation or action; "their cooperation with us was
         essential for the success of our mission" [ant:
         competition, contention, rivalry]
    2: the practice of cooperating; "economic cooperation"; "they
       agreed on a policy of cooperation"

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "cooperation":
   accompaniment, accord, accordance, advocacy, affiliation, affinity,
   agreement, aid, alliance, alternation, assent, assistance,
   association, auspices, backing, backup, battledore and shuttlecock,
   blessing, cahoots, chorus, co-working, coaction, coherence,
   coincidence, collaboration, collective farm, collectivism,
   collectivity, collegiality, collusion, combination,
   combined effort, common ownership, communal effort, communion,
   communism, community, commutation, companionship, company,
   compatibility, complementary distribution, concert,
   concerted action, concomitance, concord, concordance, concourse,
   concurrence, confluence, conformance, conformation, conformity,
   congeniality, congruence, congruency, congruity, conjunction,
   consilience, consistency, consociation, consonance, consort,
   conspiracy, cooperative society, correspondence, counterchange,
   cross fire, democracy, engagement, equivalence, exchange, favor,
   fellowship, fraternity, fraternization, friendship, give-and-take,
   harmony, help, helping hand, interaction, interchange,
   intercommunication, intercourse, interlacing, intermeshing,
   intermutation, interplay, intersection, intertwining, interweaving,
   interworking, junction, kibbutz, kolkhoz, lex talionis,
   measure for measure, membership, mesh, meshing, mutual admiration,
   mutual support, mutual transfer, mutuality, oneness, overlap,
   parallelism, parasitism, partaking, participation, patronage,
   peace, permutation, profit sharing, public ownership, quid pro quo,
   rapport, reciprocality, reciprocation, reciprocity, retaliation,
   saprophytism, seesaw, self-consistency, sharecropping, sharing,
   simultaneity, socialism, society, something for something,
   sponsorship, state ownership, support, symbiosis, symmetry, sync,
   synchronism, synergism, synergy, tally, teamwork, timing,
   tit for tat, town meeting, transposal, transposition, uniformity,
   union, unison, unisonance, united action