What does the word slab mean?

Usage examples for slab

  1. If we can only scratch the earth- bed away from under, the slab might drop a little. – The Voyages of Doctor Dolittle by Hugh Lofting
  2. I left it on the marble slab in the wash room and when I came back for it not three minutes later it was gone. – Molly Brown's Junior Days by Nell Speed