Efficacy, n.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for efficacy; he lists it under power.
Where he writes about it:
Efficacy is the power to produce an intended effect as shown in the production of it; as, the efficacy of a drug. — from power