rational number, in arithmetic, a number that can be represented as the quotient p/q of two integers such that q ≠ 0. In addition to all the fractions, the set of rational numbers includes all the integers, each of which can be written as a quotient with the integer as the numerator and 1 as the denominator. In decimal form, rational numbers are either terminating or repeating decimals. For example, 1/7 = 0., where the bar over 142857 indicates a pattern that repeats forever.
A real number that cannot be expressed as a quotient of two integers is known as an irrational number.