noun

• (historical) The boss of a shield, at or near the middle and usually projecting, sometimes in a sharp spike.
• (biology) A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane.
• (anatomy) An inward projection of the tympanic membrane of the ear.
• (zoology) One of the lateral prominences just above the hinge of a bivalve shell.
• (mycology) A bump or protrusion on the cap of a mushroom or toadstool.