Source : Webster's Revised Unabridged Dictionary (1913)

Efficacious \Ef`fi*ca"cious\, a. [L. eficax, -acis, fr.
   efficere. See {Effect}, n.]
   Possessing the quality of being effective; productive of, or
   powerful to produce, the effect intended; as, an efficacious
   law.

   Syn: See {Effectual}. -- {Ef`fi*ca"cious*ly}, adv. --
        {Ef`fi*ca"cious*ness}, n.

Source : WordNet®

efficaciousness
     n : capacity or power to produce a desired effect; "concern
         about the safety and efficacy of the vaccine" [syn: {efficacy}]
         [ant: {inefficacy}]