Noun

1. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
2. One of the lateral prominence just above the hinge of a bivalve shell.
3. A bump or protrusion on the top of the cap.
4. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
   • 1786: It is thirteen inches diameter, made of wood covered with leather, and an iron plate decorated with nails and mouldings; the boss or umbo projects four inches. — Francis Grose, A Treatise on Ancient Armour and Weapons, page xii.