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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

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British Dictionary definitions for efficacy


  1. the quality of being successful in producing an intended result; effectiveness
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.

Online Etymology Dictionary, © 2010 Douglas Harper