adj. 1.  Acting or producing effectively with a minimum of waste, expense, or unnecessary effort: an efficient builder; an efficient factory. 2.  Acting directly to produce an effect: the efficient cause of the revolution. 3.  Causing less waste or requiring less effort than comparable devices or methods. Used in combination: energy-efficient wind turbines; cost-efficient health care. [Middle English, from Old French, from Latin efficiēns, efficient-, present participle of efficere, to effect; see  EFFECT.] ef·ficient·ly adv.  | 
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