noun the hilum.
2. umbilicus ·noun one of foci of an ellipse, or other curve.
3. umbilicus ·noun either one of the two apertures in the calamus of a feather.
4. umbilicus ·noun a depression or opening in the center of the base of many spiral shells.
5. umbilicus ·noun an ornamented or painted ball or boss fastened at each end of the stick on which manuscripts were rolled.
6. umbilicus ·noun 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.
7. umbilicus ·noun 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.