Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

   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.

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

   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. )

Source : WordNet®

denominator
     n : the divisor of a fraction