umbo

noun

1. 1

   A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane. For example the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.

2. 2

   One of the lateral prominences just above the hinge of a bivalve shell.

3. 3

   A bump or protrusion on the top of the cap.

4. 4

   The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.