borrowed from Middle French efficace "effective" or its source Latin efficāc-, efficāx "capable of fulfilling a function, effective" (from efficere "to make, bring about, produce, carry out" + -āc-, -āx, deverbal suffix denoting habitual or successful performance) + -ious
adjective
having the power to produce a desired effect
an efficacious remedy
effectiveeffectualefficientefficacious mean producing or capable of producing a result. effective stresses the actual production of or the power to produce an effect. effectual suggests the accomplishment of a desired result especially as viewed after the fact.