Source : Webster's Revised Unabridged Dictionary (1913)

Efficient \Ef*fi"cient\, a. [L. efficiens, -entis, p. pr. of
   efficere to effect: cf. F. efficient. See {Effect}, n.]
   Causing effects; producing results; that makes the effect to
   be what it is; actively operative; not inactive, slack, or
   incapable; characterized by energetic and useful activity;
   as, an efficient officer, power.

         The efficient cause is the working cause. --Wilson.

   Syn: Effective; effectual; competent; able; capable;
        material; potent.

Efficient \Ef*fi"cient\, n.
   An efficient cause; a prime mover.

         God . . . moveth mere natural agents as an efficient
         only.                                    --Hooker.

Source : WordNet®

efficient
     adj 1: being effective without wasting time or effort or expense;
            "an efficient production manager"; "efficient engines
            save gas" [ant: {inefficient}]
     2: able to accomplish a purpose; functioning effectively;
        "people who will do nothing unless they get something out
        of it for themselves are often highly effective
        persons..."-G.B.Shaw; "effective personnel"; "an efficient
        secretary"; "the efficient cause of the revolution" [syn:
        {effective}]