\Um"bo\, n.; pl. L. {Umbones}, E. {Umbos}. [L.]
1. The boss of a shield, at or near the middle, and usually
   projecting, sometimes in a sharp spike.
2. A boss, or rounded elevation, or a corresponding
   depression, in a palate, disk, or membrane; as, the umbo
   in the integument of the larv[ae] of echinoderms or in the
   tympanic membrane of the ear.
3. (Zo["o]l.) One of the lateral prominence just above the
   hinge of a bivalve shell.