The Collaborative International Dictionary of English v.0.48:

Umbo \Um"bo\, n.; pl. L. Umbones, E. Umbos. [L.]
   1. The boss of a shield, at or near the middle, and usually
      projecting, sometimes in a sharp spike.
      [1913 Webster]

   2. 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.
      [1913 Webster]

   3. (Zool.) One of the lateral prominences just above the
      hinge of a bivalve shell.
      [1913 Webster] Umbonate

WordNet (r) 3.0 (2006):

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