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 larvae of echinoderms or in the tympanic membrane of the ear.
  3. One of the lateral prominence just above the hinge of a bivalve shell.