# Clarification on $M^2$ factor

In beam optics, the $$M^2$$ factor is a "catch-all" single number that describes how "good" a beam is, i.e., how close it is to an ideal Gaussian beam, which by definition has $$M^2 = 1$$. This encyclopedia article says that "A laser beam is often said to be "$$M^2$$ times diffraction-limited"." But what does that mean? I assume it is as simple as having a beam with e.g. $$M^2 = 2$$ grow twice as fast in diameter, all other things (wavelength, beam waist) being the same? Or is there more to it?

On a related note, how good of a description is the $$M^2$$ factor in practice? The linked article talks about how a single number simply cannot describe all qualities of a beam -- and I agree with that --, but would a simple description with the $$M^2$$ factor really be that bad?