- 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.
efficacy definition by Webster's New World
Origin: Middle English and amp; Old French efficace ; from Classical Latin efficacia ; from efficax: see efficacious
efficacy definition by American Heritage Dictionary
Origin: Latin efficācia, from efficāx, efficāc-, efficacious; see efficacious.