1. the navel , Lat. umbilicus, (Iliad by Homer), etc.

2. anything central (like a navel) :

3. the knob or boss in the middle of the shield, Lat. umbo, (Iliad by Homer)

4. a button or knob on the horse's yoke to fasten the reins to, (Iliad by Homer)

5. in plural the knobs at each end of the stick round which books were rolled , Lat. umbilici, (Lucian)

6. the centre or middle point , as the island of Calypso is the ὀμφαλός of the sea, (Odyssey by Homer); and Delphi (or rather a round stone in the Delphic temple) was called ὀμφαλός as marking the middle point of Earth , (Pindar), etc. (ML)