efficience /(ĕf*fĭsh"ens)/

ef·fi·cience

efficience

n.
  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.
  2. The ratio of useful work to energy expended. (Mech.)

Phrases & Compounds
efficiency of a heat engine
the ratio of the work done by an engine, to the work due to the heat supplied to it.