What does "slab board" mean?

I've noticed in higher-priced eBay auction descriptions the term "Slab" or "Slab Board". What exactly is that? Thanks.

It refers to no radius on the fret board?

I thought it meant the bottom of the fingerboard where it was glued to the neck is flat and rather thick. One of those phrases that is oft used but not real intuitive.

The first few years rosewood fingerboards were made, the top of the maple neck was flat and the rosewood was glued to the flat surface and curved to the radius on top where the frets were. In '62 the maple was curved on top to the radius and the rosewood was fitted to the curved maple. Therefore, the rosewood was much thicker and round on top and flat on the bottom on the slab and only a thinner verneer of rosewood after that. BTW, all necks have some degree of radius.

That straightens me out. I had been told the previous long ago yet I have never seen a flat fender board. Makes sense.
Thanks Jake