Efficacy is defined as the ability to do what is defined as desired or to be effective at producing a result.(noun)
An example of efficacy is when a crime bill is effective at stopping crime.
See efficacy in Webster's New World College Dictionary
Origin: ME & OFr efficace < L efficacia < efficax: see efficacious
See efficacy in American Heritage Dictionary 4
Origin: Latin efficācia
Origin: , from efficāx, efficāc-, efficacious; see efficacious.
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