The Collaborative International Dictionary of English v.0.48:

Denominator \De*nom"i*na`tor\, n. [Cf. F. d['e]nominateur.]
   1. One who, or that which, gives a name; origin or source of
      a name.
      [1913 Webster]

            This opinion that Aram . . . was the father and
            denomination of the Syrians in general. --Sir W.
                                                  Raleigh.
      [1913 Webster]

   2. (Arith.) That number placed below the line in common
      fractions which shows into how many parts the integer or
      unit is divided.
      [1913 Webster]

   Note: Thus, in 3/5, 5 is the denominator, showing that the
         integer is divided into five parts; and the numerator,
         3, shows how many parts are taken.
         [1913 Webster]

   3. (Alg.) That part of any expression under a fractional form
      which is situated below the horizontal line signifying
      division.
      [1913 Webster]

   Note: In this sense, the denominator is not necessarily a
         number, but may be any expression, either positive or
         negative, real or imaginary. --Davies & Peck (Math.
         Dict.)
         [1913 Webster]

   common denominator a number which can divide either of two
      or more other numbers without leaving a remainder in any
      of the divisions; as, 2 and 4 are common denominators of
      12 and 28..

   greatest common denominator the largest common
      denominator of two or more numbers; as, 9 is the greatest
      common denominator of 18 and 27..
      [PJC]

WordNet (r) 3.0 (2006):

denominator
    n 1: the divisor of a fraction