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] efficience
From 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]