Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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