Pronunciation: (um'bō),[key] —n., —pl.um•bo•nesPronunciation: (um-bō'nēz),[key]um•bos. 1. a boss on a shield, as one at the center of a circular shield.
2. any similar boss or protuberance.
3. Zool.the beak of a bivalve shell; the protuberance of each valve above the hinge.
4. Anat.the depressed area on the outer surface of the tympanic membrane.
5. a blunt or rounded protuberance arising from a surface, as on a pine cone scale.