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. Milton. [1913 Webster]

Syn. -- Virtue; force; energy; potency; efficiency.

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ROGET THESAURUS

Efficacy

Utility

N utility, usefulness, efficacy, efficiency, adequacy, service, use, stead, avail, help, applicability, subservience, function, value, worth, money's worth, productiveness, cui bono, utilization, step in the right direction, common weal, commonwealth public good, public interest, utilitarianism, useful, of use, serviceable, proficuous, good for, subservient, conducive, subsidiary, advantageous, profitable, gainful, remunerative, worth one's salt, valuable, prolific, adequate, efficient, efficacious, effective, effectual, expedient, applicable, available, ready, handy, at hand, tangible, commodious, adaptable, of all work, usefully, pro bono publico.

Power

N power, potency, potentiality, jiva, puissance, might, force, energy, dint, right hand, right arm, ascendency, sway, control, prepotency, prepollence, almightiness, omnipotence, authority, strength, ability, ableness, competency, efficacy, efficiency, productivity, expertise (skill), validity, cogency, enablement, vantage ground, influence, pressure, conductivity, elasticity, gravity, electricity, magnetism, galvanism, voltaic electricity, voltaism, electromagnetism, atomic power, nuclear power, thermonuclear power, fuel cell, hydraulic power, water power, hydroelectric power, solar power, solar energy, solar panels, tidal power, wind power, attraction, vis inertiae, vis mortua, vis viva, potential energy, dynamic energy, dynamic friction, dynamic suction, live circuit, live rail, live wire, capability, capacity, quid valeant humeri quid ferre recusent, faculty, quality, attribute, endowment, virtue, gift, property, qualification, susceptibility, powerful, puissant, potential, capable, able, equal to, up to, cogent, valid, efficient, productive, effective, effectual, efficacious, adequate, competent, multipotent, plenipotent, omnipotent, almighty, forcible, (energetic), influential, productive, powerfully, by virtue of, by dint of, a toute force, dos moi pou sto kai kino ten gen, eripuit coelo fulmen sceptrumque tyrannis, fortis cadere cedere non potest.

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