*denominator*

n. [ Cf. F. dénominateur. ] 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 ]

☞ 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 ]

☞ 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 ]