\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.