a. [ L. eficax, -acis, fr. efficere. See Effect, n. ] Possessing the quality of being effective; productive of, or powerful to produce, the effect intended; as, an efficacious law.
Syn. -- See Effectual.
-- Ef`fi*ca"cious*ly, adv. [1913 Webster]
n. the capacity or power to produce a desired effect. Syn. -- efficacy. [ WordNet 1.5 ]