can be consider as the cross session diameter of the beam. However, light does not have shape, so how do you really define what cross session is. Beam waist is defined for Gaussian beam, where a beam of light whose transverse em field intensity can be approximated with Gaussian distribution. The width is the radius where the intensity and field amplitude drop to 1/e or 1/e^2. Since all beams diverge, and at some point along the axis of propagation, there is a point where the width is minimum, and that is the bean waist. This is pretty much all I know about it.