umbo

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The boss or knob at the center of a shield.
2. n. Biology A knoblike protuberance arising from a surface, as the prominence near the hinge of a bivalve shell or the projection at the scale tip of a seed-bearing cone.
3. n. Anatomy A small projection at the center of the outer surface of the eardrum.

Century Dictionary and Cyclopedia

1. n. The boss of a shield, central in the case of a circular shield. The umbo was sometimes hollow, convex toward the outer side and within allowing the hand to pass into the hollow and grasp a transverse bar; this form occurs especially in small round shields (see buckler); sometimes the umbo terminated in a spike which was a formidable weapon of offense.
2. n. A boss or knob. In botany, the knob in the center of the pileus or cap of an agaricoid fungus.

Wiktionary

1. n. biology A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
2. n. zoology One of the lateral prominence just above the hinge of a bivalve shell.
3. n. mycology A bump or protrusion on the top of the cap.
4. n. historical The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.

GNU Webster's 1913

1. n. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
2. n. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane.
3. n. (Zoöl.) One of the lateral prominences just above the hinge of a bivalve shell.

WordNet 3.0

1. n. a slight rounded elevation where the malleus attaches to the eardrum