\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. )