Source : Webster's Revised Unabridged Dictionary (1913)

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: Virtue; force; energy; potency; efficiency.

Source : WordNet®

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