The Collaborative International Dictionary of English v.0.48:

Conclusive \Con*clu"sive\, a. [Cf. F. conclusif.]
   Belonging to a close or termination; decisive; convincing;
   putting an end to debate or question; leading to, or
   involving, a conclusion or decision.
   [1913 Webster]

         Secret reasons . . . equally conclusive for us as they
         were for them.                           --Rogers.
   [1913 Webster]

   Conclusive evidence (Law), that of which, from its nature,
      the law allows no contradiction or explanation.

   Conclusive presumption (Law), an inference which the law
      makes so peremptorily that it will not allow it to be
      overthrown by any contrary proof, however strong.

   Syn: Final; ultimate; unanswerable. See Final.
        [1913 Webster]

WordNet (r) 3.0 (2006):

conclusive
    adj 1: forming an end or termination; especially putting an end
           to doubt or question; "conclusive proof"; "the evidence
           is conclusive" [ant: inconclusive]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "conclusive":
   absolute, adducible, admissible, admitting no exception, all-out,
   apodictic, assuring, attestative, attestive, authentic,
   authoritative, based on, binding, bound, boundary, categorical,
   caudal, certain, circumstantial, clear, clear and distinct,
   clear as day, complete, completing, completive, completory,
   compulsory, concluding, consummative, convictional, convincing,
   crowning, culminating, cumulative, damning, decided, decisive,
   decretory, definite, definitive, determinate, determinative,
   dictated, documentary, documented, downright, endmost, entailed,
   entire, eventual, evidential, evidentiary, ex parte, explicit,
   express, extreme, eye-witness, factual, farthest, final,
   finalizing, finishing, firsthand, fixed, flat, flat-out,
   founded on, fulfilling, global, grounded on, hard-and-fast,
   hearsay, imperative, implicit, imposed, impressive, inappealable,
   incontrovertible, indicative, indisputable, ineluctable,
   inevitable, irrefutable, irresistible, irrevocable, last, limiting,
   mandated, mandatory, material, must, necessary, nuncupative,
   obligatory, out-and-out, outright, overwhelming, peremptory,
   perfect, perfectly sure, persuasive, polar, positive, predestined,
   predetermined, prescript, prescriptive, presumptive, probative,
   reliable, required, round, satisfactory, satisfying, significant,
   straight, straight-out, suggestive, sure, sure-enough, symptomatic,
   tail, telling, terminal, terminating, terminative, total, true,
   ultimate, unambiguous, uncircumscribed, unconditional,
   unconditioned, undoubting, unequivocal, unhampered, unhesitating,
   univocal, unlimited, unmistakable, unmitigated, unqualified,
   unquestioning, unreserved, unrestricted, unwaivable, utter, valid,
   weighty, whole, without appeal, without exception,
   without reserve



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CONCLUSIVE. What puts an end to a thing. A conclusive presumption of law, is
one which cannot be contradicted even by direct and positive proof. Take,
for example, the presumption that an infant is incapable of judging whether
it is or is not against his interest; When infancy is pleaded and proved,
the plaintiff cannot show that the defendant was within one day of being of
age when the contract was made, and perfectly competent to make a contract.
3 Bouv. Inst. n. 3061.