The Collaborative International Dictionary of English v.0.48:

Umbilicus \Um`bi*li"cus\, n. [L. See Umbilic.]
   1. (Anat.) The depression, or mark, in the median line of the
      abdomen, which indicates the point where the umbilical
      cord separated from the fetus; the navel; the belly
      button, in humans.
      [1913 Webster]

   2. (Gr. & Rom. Antiq.) An ornamented or painted ball or boss
      fastened at each end of the stick on which manuscripts
      were rolled. --Dr. W. Smith.
      [1913 Webster]

   3. (Bot.) The hilum.
      [1913 Webster]

   4. (Zool.)
      (a) A depression or opening in the center of the base of
          many spiral shells.
      (b) Either one of the two apertures in the calamus of a
          feather.
          [1913 Webster]

   5. (Geom.)
      (a) One of the foci of an ellipse, or other curve. [Obs.]
      (b) A point of a surface at which the curvatures of the
          normal sections are all equal to each other. A sphere
          may be osculatory to the surface in every direction at
          an umbilicus. Called also umbilic.
          [1913 Webster]

WordNet (r) 3.0 (2006):

umbilicus
    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]

Moby Thesaurus II by Grady Ward, 1.0:

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