The Collaborative International Dictionary of English v.0.48:

Navel \Na"vel\ (n[=a]"v'l), n. [AS. nafela, fr. nafu nave; akin
   to D. navel, G. nabel, OHG. nabolo, Icel. nafli, Dan. navle,
   Sw. nafle, L. umbilicus, Gr. 'omfalo`s, Skr. n[=a]bh[imac]la.
   [root]260. See Nave hub, and cf. Omphalic, Nombril,
   Umbilical.]
   1. (Anat.) A mark or depression in the middle of the abdomen;
      the umbilicus; called also belly button in humans. See
      Umbilicus.
      [1913 Webster]

   2. The central part or point of anything; the middle.
      [1913 Webster]

            Within the navel of this hideous wood,
            Immured in cypress shades, a sorcerer dwells.
                                                  --Milton.
      [1913 Webster]

   3. (Gun.) An eye on the under side of a carronade for
      securing it to a carriage.
      [1913 Webster]

   Navel gall, a bruise on the top of the chine of the back of
      a horse, behind the saddle. --Johnson.

   Navel point. (Her.) Same as Nombril.
      [1913 Webster]

WordNet (r) 3.0 (2006):

navel
    n 1: a scar where the umbilical cord was attached; "you were not
         supposed to show your navel on television"; "they argued
         whether or not Adam had a navel"; "she had a tattoo just
         above her bellybutton" [syn: navel, umbilicus,
         bellybutton, belly button, omphalos, omphalus]
    2: the center point or middle of something; "the Incas believed
       that Cuzco was the navel of the universe" [syn: navel,
       navel point]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "navel":
   axis, center, center of action, center of gravity, centroid,
   centrum, core, dead center, epicenter, heart, hub, kernel, marrow,
   medulla, metacenter, middle, nave, nub, nucleus, omphalos, pith,
   pivot, storm center, umbilicus