- The definition of efficacious is someone or something that successfully produced a result that was desired.
An example of efficacious is a program that was designed to help stop crime and that reduced the crime rate by 80 percent.
Origin of efficaciousClassical Latin efficax (gen. efficacis) ; from efficere, to bring to pass, accomplish (see effect) + -ous
Origin of efficaciousFrom Latin efficāx, efficāc-, from efficere, to effect; see effect.
(comparative more efficacious, superlative most efficacious)