Rational Numbers

Definition: a rational number is any number that can be written in the p/q form of two integers, p and q, with the denominator q not equal to zero. The set of all rational numbers is denoted by $\mathbb{Q}$, i.e.,

$\qquad\quad\mathbb{Q} = \left\{ \dfrac{p}{q} \;\big|\;\; p, q\;\in\;\mathbb{Z};\;\; q\neq 0\right\}$

In decimal representation, a number is a rational number if and only if the decimal expansion of a number either terminates after a finite number of digits or begins to repeat the same finite sequence of digits over and over. This holds true irrespective of the base, i.e, decimal, binary, etc.

Related:
Integers (definition)