n. - The boss of a shield, at or near the middle, and usually
projecting, sometimes in a sharp spike.
n. - A boss, or rounded elevation, or a corresponding depression,
in a palate, disk, or membrane; as, the umbo in the integument of the
larvae of echinoderms or in the tympanic membrane of the ear.
n. - One of the lateral prominence just above the hinge of a
bivalve shell.