Source : WordNet®

irrational number
     n : a real number that cannot be expressed as a rational number

Source : Free On-Line Dictionary of Computing

irrational number
     
         A {real number} which is not a {rational
        number}, i.e. it is not the ratio of two {integer}s.
     
        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)