*ellipsoid*

n. [ Ellipse + -oid: cf. F. ellipsoide. ] (Geom.) A solid, all plane sections of which are ellipses or circles. See Conoid, n., 2 (a). [ 1913 Webster ]

☞ The ellipsoid has three principal plane sections, a, b, and c, each at right angles to the other two, and each dividing the solid into two equal and symmetrical parts. The lines of meeting of these principal sections are the axes, or principal diameters of the ellipsoid. The point where the three planes meet is the center. [ 1913 Webster ]

Ellipsoid of revolution, a spheroid; a solid figure generated by the revolution of an ellipse about one of its axes. It is called a prolate spheroid, or prolatum, when the ellipse is revolved about the major axis, and an oblate spheroid, or oblatum, when it is revolved about the minor axis.