The Collaborative International Dictionary of English v.0.48:

Definitive \De*fin"i*tive\, n. (Gram.)
   A word used to define or limit the extent of the
   signification of a common noun, such as the definite article,
   and some pronouns.
   [1913 Webster]

   Note: Definitives . . . are commonly called by grammarians
         articles. . . . They are of two kinds, either those
         properly and strictly so called, or else pronominal
         articles, such as this, that, any, other, some, all,
         no, none, etc. --Harris (Hermes).
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Definitive \De*fin"i*tive\, a. [L. definitivus: cf. F.
   d['e]finitif.]
   1. Determinate; positive; final; conclusive; unconditional;
      express.
      [1913 Webster]

            A strict and definitive truth.        --Sir T.
                                                  Browne.
      [1913 Webster]

            Some definitive . . . scheme of reconciliation.
                                                  --Prescott.
      [1913 Webster]

   2. Limiting; determining; as, a definitive word.
      [1913 Webster]

   3. Determined; resolved. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

definitive
    adj 1: clearly defined or formulated; "the plain and unequivocal
           language of the laws"- R.B.Taney [syn: definitive,
           unequivocal]
    2: of recognized authority or excellence; "the definitive work
       on Greece"; "classical methods of navigation" [syn:
       authoritative, classical, classic, definitive]
    3: supplying or being a final or conclusive settlement; "a
       definitive verdict"; "a determinate answer to the problem"
       [syn: definitive, determinate]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "definitive":
   absolute, accurate, actual, admitting no exception, all-out,
   arrant, authoritative, boundary, categorical, caudal, clarifying,
   classical, clean-cut, clear, clear-cut, closing, complete,
   completing, concluding, conclusive, confining, consummate, crass,
   decided, decisive, defining, definite, determinate, determinative,
   determining, downright, egregious, ending, endmost, entire,
   eventual, exact, exclusive, exhaustive, explicit, express, extreme,
   farthest, final, finishing, fixed, flagrant, flat, flat-out,
   glaring, global, gross, implicit, inappealable, indisputable,
   intolerable, last, limitary, limitative, limiting, out-and-out,
   outright, peremptory, perfect, polar, positive, precious, precise,
   profound, pronounced, proper, rank, real, regular, reliable,
   restricting, restrictive, round, settling, shattering, shocking,
   specific, stark, stark-staring, straight, straight-out,
   superlative, surpassing, tail, terminal, terminating, terminative,
   the veriest, thorough, thoroughgoing, through and through, total,
   ultimate, unambiguous, unbearable, uncircumscribed, unconditional,
   unconditioned, unconscionable, undeniable, undoubting, unequivocal,
   unhampered, unhesitating, unlimited, unmistakable, unmitigated,
   unqualified, unquestioning, unrelieved, unreserved, unrestricted,
   unspoiled, unwaivable, utter, whole, without exception,
   without reserve