1. the divisor of a fraction;

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