Webster's English Dictionary

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efficiency

ef.fi.cien.cy \i-'fish-*n-se-\ n 1: the quality or degree of being 
   efficient  2a: efficient operation  2b1: effective operation as measured by 
   a comparison of production with cos t (as in energy, time, and money) 2b2: 
   the ratio of the useful energy delivered by a dynamic system to the en ergy 
   supplied to it