Enlightened by the degree of paraxiality that was recently introduced by Gawhary and Severini in Opt.Lett.33,1360(2008),a parameter,called the degree of diffraction,is defined to describe the diffractive spreading of a monochromatic light beam.This is a parameter that is independent of the degree of paraxiality.Nevertheless,the same as the degree of paraxiality,the degree of diffraction depends only on the angular spectrum of a beam.With this definition,it is ready to compare quantitatively the diffractive spreading of different beams.-WUJie|jiewu@shu.edu.cn