The Collaborative International Dictionary of English v.0.48:

Decisive \De*ci*sive\, a. [Cf. F. d['e]cisif. See Decision.]
   1. Having the power or quality of deciding a question or
      controversy; putting an end to contest or controversy;
      final; conclusive. "A decisive, irrevocable doom."
      --Bates. "Decisive campaign." --Macaulay. "Decisive
      proof." --Hallam.
      [1913 Webster]

   2. Marked by promptness and decision.
      [1913 Webster]

            A noble instance of this attribute of the decisive
            character.                            --J. Foster.

   Syn: Decided; positive; conclusive. See Decided. --
        De*ci"sive*ly, adv. -- De*ci"sive*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

decisiveness
    n 1: the trait of resoluteness as evidenced by firmness of
         character or purpose; "a man of unusual decisiveness" [syn:
         decisiveness, decision] [ant: indecision,
         indecisiveness]
    2: the quality of being final or definitely settled; "the
       finality of death" [syn: finality, conclusiveness,
       decisiveness] [ant: inconclusiveness]