EfficienceEf*fi"cience\, Efficiency \Ef*fi"cien*cy\, 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 an engine, to the work due to the heat supplied to it.