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From 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]