Definitions
1.nounA 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.nounOne of the lateral prominences just above the hinge of a bivalve shell.
3.nounA bump or protrusion on the top of the cap.
4.nounThe boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.