- Efficacy is defined as the ability to do what is defined as desired or to be effective at producing a result.
An example of efficacy is when a crime bill is effective at stopping crime.
Origin: Middle English and amp; Old French efficace ; from Classical Latin efficacia ; from efficax: see efficacious
Used by arrangement with John Wiley & Sons, Inc.
Origin: Latin efficācia, from efficāx, efficāc-, efficacious; see efficacious.