By analysing the limitation of the second moment definition of divergence angle and redefining this angle which is related to the beam width, we pointed out that the quality factor M2 is essentially a measure of the beam collimation. Taking the nonlinear self focusing optics phenomenon as an example, we illustrated that this factor could be smaller than one if the possible nonlinear effects become important. The discrepancy between the inequality M2≥1 in linear optics and the uncertainty relation of quantum mechanics was also emphasized.