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):

decisive
    adj 1: determining or having the power to determine an outcome;
           "cast the decisive vote"; "two factors had a decisive
           influence" [ant: indecisive]
    2: unmistakable; "had a decisive lead in the polls"
    3: characterized by decision and firmness; "an able and decisive
       young woman"; "we needed decisive leadership"; "she gave him
       a decisive answer" [ant: indecisive]
    4: forming or having the nature of a turning point or crisis; "a
       critical point in the campaign"; "the critical test" [syn:
       critical, decisive]

Moby Thesaurus II by Grady Ward, 1.0:

193 Moby Thesaurus words for "decisive":
   absolute, adducible, admissible, admitting no exception, alert,
   all-out, apodictic, apt, assured, assuring, attestative, attestive,
   authentic, authoritative, based on, behind the scenes, bent,
   binding, bound, bound and determined, categorical, causal,
   causative, certain, charged, circumstantial, clear,
   clear and distinct, clear as day, climacteric, committed, complete,
   compulsory, conclusive, constitutive, convictional, convincing,
   critical, crucial, cumulative, damning, decided, decretory,
   dedicated, definite, definitive, determinate, determinative,
   determined, devoted, dictated, documentary, documented, downright,
   earnest, effectual, emergent, entailed, entire, etiological,
   evidential, evidentiary, ex parte, exigent, expeditious, explicit,
   express, eye-witness, factual, final, firsthand, fixed, flat,
   flat-out, formative, founded on, global, grounded on,
   hard-and-fast, hearsay, immediate, imperative, imperious, implicit,
   imposed, impressive, inappealable, incontrovertible, indicative,
   indisputable, ineluctable, inevitable, instant, instantaneous,
   institutive, intent, irrefutable, irresistible, irrevocable,
   kairotic, loaded, mandated, mandatory, masterful, material, must,
   necessary, nuncupative, obligatory, obstinate, occasional,
   originative, out-and-out, outright, overwhelming, peremptory,
   perfect, perfectly sure, persevering, persistent, persuasive,
   pivotal, positive, predestined, predetermined, pregnant, prescript,
   prescriptive, presumptive, probative, prompt, punctual, purposeful,
   quick, ready, relentless, reliable, required, resolute, resolved,
   round, satisfactory, satisfying, self-assured, self-confident,
   serious, set, settled, significant, sincere, single-minded, speedy,
   steadfast, straight, straight-out, suggestive, summary, sure,
   sure-enough, swift, symptomatic, telling, tenacious, total, true,
   ultimate, unambiguous, uncircumscribed, unconditional,
   unconditioned, undoubting, unequivocal, unhampered, unhesitating,
   univocal, unlimited, unmistakable, unmitigated, unqualified,
   unquestioning, unreserved, unrestricted, unswerving, unwaivable,
   unwavering, utter, valid, weighty, whole, wholehearted,
   without appeal, without exception, without reserve