The definition of efficacious is someone or something that successfully produced a result that was desired.(adjective)
An example of efficacious is a program that was designed to help stop crime and that reduced the crime rate by 80 percent.
See efficacious in Webster's New World College Dictionary
Origin: L efficax (gen. efficacis) < efficere, to bring to pass, accomplish (see effect) + -ous
See efficacious in American Heritage Dictionary 4
Origin: From Latin efficāx, efficāc-
Origin: , from efficere, to effect; see effect.
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