The Collaborative International Dictionary of English v.0.48:

Determinative \De*ter"mi*na*tive\, n.
   That which serves to determine.
   [1913 Webster]

         Explanatory determinatives . . . were placed after
         words phonetically expressed, in order to serve as an
         aid to the reader in determining the meaning. --I.
                                                  Taylor (The
                                                  Alphabet).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Determinative \De*ter"mi*na*tive\, a. [Cf. F. d['e]terminatif.]
   Having power to determine; limiting; shaping; directing;
   conclusive.
   [1913 Webster]

         Incidents . . . determinative of their course. --I.
                                                  Taylor.
   [1913 Webster]

   Determinative tables (Nat. Hist.), tables presenting the
      specific character of minerals, plants, etc., to assist in
      determining the species to which a specimen belongs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

determinative
    adj 1: having the power or quality of deciding; "the crucial
           experiment"; "cast the deciding vote"; "the determinative
           (or determinant) battle" [syn: deciding(a),
           determinant, determinative, determining(a)]
    n 1: one of a limited class of noun modifiers that determine the
         referents of noun phrases [syn: determiner,
         determinative]
    2: a determining or causal element or factor; "education is an
       important determinant of one's outlook on life" [syn:
       determinant, determiner, determinative, determining
       factor, causal factor]