Efficience, Efficiency, n.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for efficiency; he lists it under power.
Where he writes about it:
Efficiency is active power to effect a definite result, the power that actually does, as distinguished from that which may do. — from power
Efficiency is effectual agency, competent power; efficiency is applied in mechanics as denoting the ratio of the effect produced to the power expended in producing it; but this word is chiefly used of intelligent agents as denoting the quality that brings all one's power to bear promptly and to the best purpose on the thing to be done. — from power