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]