The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Effectual steps for the suppression of the rebellion.
                                                  --Macaulay.
   [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

effectual
    adj 1: 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, effectual, efficacious] [ant:
           ineffective, ineffectual, uneffective]
    2: having legal efficacy or force; "a sound title to the
       property" [syn: legal, sound, effectual]