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.