efficacyn.[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.(Milton.)()n.[L. efficientia.]1. The quality of being efficient or producing an effect or effects; efficient power; effectual agency.()The manner of this divine efficiency being far above us. (Hooker.)2. (Mech.) The ratio of useful work to energy expended.(Rankine.)efficiency of a heat engine, the ratio of the work done by an engine, to the work due to the heat supplied to it.()