Definition of: umbo

(um′bō) noun plural um·bo·nes (um·bō′nēz) or ·bos
1. The boss or projecting spike in the center of a shield.
2. Zool. An elevation, boss, or knob, as the prominence of a bivalve shell near the hinge, or the plate of an echinoderm.
3. Bot. The top of the cap of certain fungi.
4. Anat. The surface of the tympanic membrane at the point of attachment to the malleus. [<L]
—um′bo·nal, um·bon·ic (um·bon′ik) adjective