The Collaborative International Dictionary of English v.0.48:

Synergy \Syn"er*gy\, n. [Gr. ?. See Synergetic.]
   2. Combined action; especially (Med.), the combined healthy
      action of every organ of a particular system; as, the
      digestive synergy.
      [1913 Webster]

   2. An effect of the interaction of the actions of two agents
      such that the result of the combined action is greater
      than expected as a simple additive combination of the two
      agents acting separately; -- also called synergism..
      Opposite to antagonism.
      [PJC]

WordNet (r) 3.0 (2006):

synergy
    n 1: the working together of two things (muscles or drugs for
         example) to produce an effect greater than the sum of their
         individual effects [syn: synergy, synergism]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "synergy":
   accompaniment, accordance, agreement, alliance, association,
   bipartisanship, cahoots, co-working, coaction, coadjuvancy,
   coadministration, coagency, cochairmanship, codirectorship,
   coincidence, collaboration, 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, harmony, joining of forces,
   joint effort, joint operation, junction, mass action, morale,
   mutual assistance, mutualism, mutuality, octet, parasitism,
   pooling, pooling of resources, pulling together, quartet, quintet,
   reciprocity, saprophytism, septet, sextet, simultaneity,
   solidarity, symbiosis, synchronism, synergism, team spirit,
   teamwork, trio, triumvirate, troika, union, united action