Definition:Rational Number/Fraction/Denominator

Definition
Let $\dfrac a b$ be a fraction. The term $b$ is known as the denominator of $\dfrac a b$.

Also see

 * Definition:Division/Divisor

Linguistic Note
The term denominator derives from the word denomination which, in this context, means grouping of elements of the same kind.

Thus, in the fraction $\dfrac 3 4$, it states that the objects of the same kind in question (of which the numerator specifies that there are three) are quarters.