noun, plural efficiencies.
the state or quality of being efficient; competency in performance.
accomplishment of or ability to accomplish a job with a minimum expenditure of time and effort: The assembly line increased industry's efficiency.
the ratio of the work done or energy developed by a machine, engine, etc., to the energy supplied to it, usually expressed as a percentage.

1585–95; < Latin efficientia, equivalent to efficient- (see efficient) + -ia -y3

nonefficiency, noun
superefficiency, noun, plural superefficiencies.
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World English Dictionary
efficiency (ɪˈfɪʃənsɪ)
n , pl -cies
1.  the quality or state of being efficient; competence; effectiveness
2.  See also thermal efficiency the ratio of the useful work done by a machine, engine, device, etc, to the energy supplied to it, often expressed as a percentage

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Word Origin & History

1630s, "power to accomplish something," from L. efficientia (see efficient). In mechanics, "ratio of useful work done to energy expended," from 1858. Attested from 1952 as short for efficiency apartment (itself from 1930).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

efficiency ef·fi·cien·cy (ĭ-fĭsh'ən-sē)

  1. The production of the desired effects or results with minimum waste of time, effort, or skill.

  2. A measure of effectiveness; specifically, the useful work output divided by the energy input in any system.

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Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
efficiency   (ĭ-fĭsh'ən-sē)  Pronunciation Key 
  1. The ratio of the energy delivered (or work done) by a machine to the energy needed (or work required) in operating the machine. The efficiency of any machine is always less than one due to forces such as friction that use up energy unproductively. See also mechanical advantage.

  2. The ratio of the effective or useful output to the total input in any system.

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Example sentences
Or function at all with any semblance of efficiency, effectiveness, and
This is probably not a good plan from an efficiency stand point.
Energy efficiency is the unglamorous policy that nobody seems to be taking
  seriously, but it's got superb potential.
Propellers lost efficiency and thrust when they neared supersonic speeds.
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