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]