Source : Webster's Revised Unabridged Dictionary (1913)

Effectual \Ef*fec"tu*al\ (?; 135), a. [See {Effect}, n.]
   Producing, or having adequate power or force to produce, an
   intended effect; adequate; efficient; operative; decisive.
   --Shak.

         Effectual steps for the suppression of the rebellion.
                                                  --Macaulay.

   {Effectual calling} (Theol.), a doctrine concerning the work
      of the Holy Spirit in producing conviction of sin and
      acceptance of salvation by Christ, -- one of the five
      points of Calvinism. See {Calvinism}.

   Syn: {Effectual}, {Efficacious}, {Effective}.

   Usage: An efficacious remedy is had recourse to, and proves
          effective if it does decided good, effectual if it
          does all the good desired. --C. J. Smith.

Source : WordNet®

effectual
     adj 1: sufficient to produce a result; "a man to whom painting was
            but another and less effectual way of writing dramas
            or novels or history" [ant: {ineffectual}]
     2: producing or capable of producing an intended result or
        having a striking effect; "an air-cooled motor was more
        effective than a witch's broomstick for rapid
        long-distance transportation"-LewisMumford; "effective
        teaching methods"; "effective steps toward peace"; "made
        an effective entrance"; "his complaint proved to be
        effectual in bringing action"; "an efficacious law" [syn:
        {effective}, {efficacious}] [ant: {ineffective}]
     3: exerting or capable of exerting strong physiological or
        chemical effects; "an effectual vaccine"