noun

• (anatomy) navel
• (botany) hilum
• (zoology) A depression or opening in the center of the base of many spiral shells.
• (zoology) Either of the two apertures in the calamus of a feather.
• (space science) A tube connecting an astronaut or spacecraft to the mothership, through which supplies and samples can be transferred.
• (geometry, obsolete) One of the foci of an ellipse or other curve.
• (geometry) A point of a surface at which the curvatures of the normal sections are all equal to each other.
• (historical) An ornamented or painted ball or boss fastened at each end of the stick on which manuscripts were rolled.