From 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] efficienceFrom 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]