n. pl. um·bo·nes (ŭm-bōnēz) or um·bos
1. The boss or knob at the center of a shield.
a. Biology A knoblike protuberance arising from a surface, as the prominence near the hinge of a bivalve shell or the raised area in the center of the cap of a mushroom.
b. Anatomy A small projection at the center of the outer surface of the eardrum.
[Latin umbō, umbōn-; see nobh- in the Appendix of Indo-European roots.]
umbo·nal (ŭmbə-nəl, ŭm-bōnəl) adj.
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