Umbo \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.

Source: Webster's Revised Unabridged Dictionary (1913)