umbo

Noun

umbo (plural umbones or umbos or umboes)

1. (historical) The boss of a shield, at or near the middle and usually projecting, sometimes in a sharp spike.
   • 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.1786, Francis Grose, A Treatise on Ancient Armour and Weapons, page xii
   • serves as the umbo of the region’s so-called Silicon Shield2023 March 21, Virginia Heffernan, “I Saw the Face of God in a Semiconductor Factory”, in WIRED
2. An inward projection of the tympanic membrane of the ear.
3. One of the lateral prominences just above the hinge of a bivalve shell.
4. A bump or protrusion on the cap of a mushroom or toadstool.