umbo

noun5

1. •
   A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane.
2. •
   An inward projection of the tympanic membrane of the ear.
3. •
   One of the lateral prominences just above the hinge of a bivalve shell.
4. •
   A bump or protrusion on the cap of a mushroom or toadstool.
5. •
   (historical) The boss of a shield, at or near the middle and usually projecting, sometimes in a sharp spike.

   • It is thirteen inches diameter, made of wood covered with leather, and an iron plate decorated with nails and mouldings; the boss or umbo projects four inches. — 1786