The Collaborative International Dictionary of English v.0.48:

Determinant \De*ter"mi*nant\, a. [L. determinans, p. pr. of
   determinare: cf. F. d['e]terminant.]
   Serving to determine or limit; determinative.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Determinant \De*ter"mi*nant\, n.
   1. That which serves to determine; that which causes
      determination.
      [1913 Webster]

   2. (Math.) The sum of a series of products of several
      numbers, these products being formed according to certain
      specified laws;

   Note: thus, the determinant of the nine numbers
         a, b, c,a', b', c',a'', b'', c'',
         is a b' c'' - a b'' c' + a' b'' c] - a' b c'' + a'' b'
         c. The determinant is written by placing the numbers
         from which it is formed in a square between two
         vertical lines. The theory of determinants forms a very
         important branch of modern mathematics.
         [1913 Webster]

   3. (Logic) A mark or attribute, attached to the subject or
      predicate, narrowing the extent of both, but rendering
      them more definite and precise. --Abp. Thomson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

determinant
    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: 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]
    2: the site on the surface of an antigen molecule to which an
       antibody attaches itself [syn: antigenic determinant,
       determinant, epitope]
    3: a square matrix used to solve simultaneous equations

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "determinant":
   Altmann theory, DNA, De Vries theory, Galtonian theory,
   Mendelianism, Mendelism, RNA, Verworn theory, Weismann theory,
   Weismannism, Wiesner theory, allele, allelomorph, antecedent,
   antecedents, authority, base, basis, birth, border line, bordering,
   borderline, bound, boundary, boundary condition, boundary line,
   bounding, bourn, break boundary, breakoff point, call, causation,
   cause, cause and effect, ceiling, character, chromatid, chromatin,
   chromosome, circumscription, coastal, compass, confine, cutoff,
   cutoff point, deadline, delimitation, determinative, determiner,
   determining, diathesis, division line, element, end, endowment,
   etiology, eugenics, extreme, extremity, factor, finish, floor,
   fringing, frontier, gene, genesiology, genetic code, genetics,
   ground, grounds, hedge, hereditability, heredity, heritability,
   heritage, high-water mark, inborn capacity, influence,
   inheritability, inheritance, interface, limbic, limen, liminal,
   limit, limitation, limiting, limiting factor, line,
   line of demarcation, littoral, low-water mark, lower limit, march,
   marginal, mark, matrocliny, mete, occasion, patrocliny,
   pharmacogenetics, principle, reason, recessive character,
   replication, rimming, skirting, start, starting line,
   starting point, stimulus, target date, term, terminal,
   terminal date, terminus, threshold, time allotment, upper limit,
   weight