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]