The Collaborative International Dictionary of English v.0.48:

efficacy \ef"fi*ca*cy\, n. [L. efficacia, fr. efficax. See
   Efficacious.]
   Power to produce effects; operation or energy of an agent or
   force; production of the effect intended; as, the efficacy of
   medicine in counteracting disease; the efficacy of prayer.
   "Of noxious efficacy." --Milton.

   Syn: efficacy.
        [1913 Webster]

   Syn: Virtue; force; energy; potency; efficiency.
        [1913 Webster] efficience

WordNet (r) 3.0 (2006):

efficacy
    n 1: capacity or power to produce a desired effect; "concern
         about the safety and efficacy of the vaccine" [syn:
         efficacy, efficaciousness] [ant: inefficaciousness,
         inefficacy]