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):

bellybutton
    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]