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]

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "determinative":
   absolute, adducible, admissible, antecedents, arrowhead, article,
   assuring, attestative, attestive, authentic, authoritative, base,
   based on, basis, behind the scenes, bordering, borderline,
   boundary, bounding, call, caudal, causal, causation, causative,
   cause, cause and effect, certain, character, circumstantial,
   coastal, conclusive, constitutive, convictional, convincing,
   cumulative, cuneiform, damning, decisive, definite article,
   definitive, demotic character, determinant, determiner,
   determining, documentary, documented, effectual, element, endmost,
   etiological, etiology, eventual, evidential, evidentiary, ex parte,
   extreme, eye-witness, factor, factual, farthest, final, firsthand,
   formative, founded on, fringing, frontier, grammalogue, ground,
   grounded on, grounds, hearsay, hieratic symbol, hieroglyph,
   hieroglyphic, hieroglyphics, hiragana, ideogram, ideograph,
   implicit, impressive, incontrovertible, indefinite article,
   indicative, indisputable, institutive, irrefutable, irresistible,
   kana, katakana, last, limbic, liminal, limit, limiting, littoral,
   logogram, logograph, marginal, material, noun determiner,
   nuncupative, occasion, occasional, ogham, originative,
   overwhelming, persuasive, phonetic, phonetic symbol, pictogram,
   pictograph, pivotal, polar, presumptive, principle, probative,
   radical, reliable, rimming, rune, satisfactory, satisfying,
   shorthand, significant, skirting, stimulus, suggestive, sure,
   symptomatic, tail, telling, terminal, terminating, terminative,
   threshold, ultimate, valid, wedge, weighty, word letter