The Collaborative International Dictionary of English v.0.48:

Nombril \Nom"bril\, n. [F. nombril, for OF. lombril, i. e.,
   ombril, with the article, a dim. fr. L. umbilicus the navel.
   See Navel.] (Her.)
   A point halfway between the fess point and the middle base
   point of an escutcheon; -- called also navel point. See
   Escutcheon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nombril
    n 1: the center point on a shield