American Heritage® Dictionary of the English Language, Fourth
Edition - n. The boss or knob at the center of a shield. - 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. - n. Anatomy A small
projection at the center of the outer surface of the eardrum.
Century Dictionary and Cyclopedia - 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. - n. A boss or knob. In botany, the knob in
the center of the pileus or cap of an agaricoid fungus. - 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 echi