An integer or a fraction. In mathematics, any number that can be expressed as an exact fraction (with a denominator not equal to 0), that is, as a ÷ b where a and b are integers. For example, 2/1, Ľ, 1 5/4, - 3/5 are all rational numbers, whereas p (which represents the constant 3.141592 ... ) is not. Numbers such as p are called irrational numbers.