The Collaborative International Dictionary of English v.0.44 Dictionary Definition: 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]
WordNet (r) 2.0 Dictionary Definition: efficacy
n : capacity or power to produce a desired effect; "concern
about the safety and efficacy of the vaccine" [syn: {efficaciousness}]
[ant: {inefficacy}]
