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