Webster's Revised Unabridged Dictionary · 1913

Denominator

Denominator , noun

[Compare French dénominateur.]

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

Thus, in ⅗, 5 is the denominator, showing that the integer is divided into five parts; and the numerator, 3, shows how many parts are taken.

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

In this sense, the denominator is not necessarily a number, but may be any expression, either positive or negative, real or imaginary.

Collocations (2)
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..