  A parameter, called the degree of diffraction, is defined to describe the
diffractive spreading of a monochromatic light beam. The same as the degree of
paraxiality that was introduced by Gawhary and Severini in Opt. Lett. 33, 1360
(2008), the degree of diffraction depends only on beam's angular spectrum. With
this definition, it is possible to quantitatively compare the diffractive
spreading of different light beams.
