irrational number n 1: a real number that cannot be expressed as a rational number [syn: {irrational number}, {irrational}]
irrational number <mathematics> A {real number} which is not a {rational number}, i.e. it is not the ratio of two {integers}. The decimal expansion of an irrational is infinite but does not end in an infinite repeating sequence of digits. Examples of irrational numbers are {pi}, {e} and the square root of two. (1995-04-12)