Umbo n. : The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.

Umbo n. : A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvae of echinoderms or in the tympanic membrane of the ear.

Umbo n. : One of the lateral prominence just above the hinge of a bivalve shell.

Example:
What do the concentric lines that extend outward from the Umbo represent?

Taxonomy
Umbo of tympanic membrane, free encyclopedia, shelving system, search, dome, peak, varieties, fungi, the human body, juvenile, German photographer, Otto Umbehr, co-founded, photo agency, DEPHOT, Italian designer, disambiguation pages, designer Joe Colombo.

Defination from the web:
Umbo is the (often somewhat protruding) part of a Bivalve shell which was formed when the animal was a juvenile. Umbo was the name used by the German photographer Otto Umbehr (1902-80), who co-founded the photo agency DEPHOT ...