The Collaborative International Dictionary of English v.0.48:

Synergistic \Syn`er*gis"tic\, a.
   1. Of or pertaining to synergism. "A synergistic view of
      regeneration." --Shedd.
      [1913 Webster]

   2. Cooperating; synergetic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

synergistic
    adj 1: used especially of drugs or muscles that work together so
           the total effect is greater than the sum of the two (or
           more) [syn: synergistic, interactive] [ant:
           antagonistic, incompatible]
    2: of or relating to the theological doctrine of synergism
    3: working together; used especially of groups, as subsidiaries
       of a corporation, cooperating for an enhanced effect; "a
       synergistic effect" [syn: synergetic, synergistic]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "synergistic":
   accompanying, accordant, agreeing, associate, associated,
   at one with, coacting, coactive, coadjutant, coadjuvant, coadunate,
   coefficient, coincident, collaborative, collective, collectivist,
   collectivistic, collusive, combined, combining, commensal, common,
   communal, communalist, communalistic, communist, communistic,
   communitarian, concerted, concomitant, concordant, concurrent,
   concurring, conjoint, conniving, consilient, conspiratorial,
   cooperant, cooperating, cooperative, coordinate, coworking,
   ecumenic, fellow, harmonious, harmonized, joint, meeting, mutual,
   noncompetitive, parasitic, reciprocal, saprophytic, symbiotic,
   synchronous, synergetic, synergic, uncompetitive, united,
   uniting