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]

The Collaborative International Dictionary of English v.0.48:

bellybutton \bel"ly*but`ton\, belly button \bel"ly but`ton\, n.
   The depression in the middle of the abdomen in humans left as
   a residue of the umbilical cord; the umbilicus; the navel[1].
   [PJC]

WordNet (r) 3.0 (2006):

belly button
    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]