Digital BW, The Print

Hi Everyone,

I feel funny asking this after successfully printing
in black and white for so long, but I'm still trying
to get a firm grasp on what Dmax is.  My (probably
oversimplified) understanding is that it is the
maximum darkness (density?) that a given combination
of paper and ink can achieve.

What I still don't understand is:
- how is it measured?
- what units is Dmax expressed in, or is it a
dimensionless number?

I did try googling Dmax a while back but wasn't really
satisfied with what I found, especially regarding the
two questions above.

Any additional light you can shed would be
appreciated.

Thanks,

Ken