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See more synonyms for efficacy on Thesaurus.com
noun, plural ef·fi·ca·cies.
  1. capacity for producing a desired result or effect; effectiveness: a remedy of great efficacy.
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Origin of efficacy

1520–30; < Latin efficācia, equivalent to efficāc- (see efficacious) + -ia -y3
Related formsnon·ef·fi·ca·cy, noun
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for efficacy

adequacy, effectiveness, competence, virtue, potency, vigor, use, strength, force, ability, power, efficaciousness, energy, influence, effect, weight, capability, performance, success, sufficiency

Examples from the Web for efficacy

Contemporary Examples of efficacy

Historical Examples of efficacy

British Dictionary definitions for efficacy


  1. the quality of being successful in producing an intended result; effectiveness
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for efficacy


1520s, from Latin efficacia "efficacy, efficiency," from efficax (genitive efficacis) "powerful, effective," from stem of efficere "work out, accomplish" (see effect). Earlier in same sense was efficace (c.1200), from Old French eficace (14c.), from Latin efficacia; also efficacite (early 15c.), from Latin efficacitatem.

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Online Etymology Dictionary, © 2010 Douglas Harper