ellipsoid

Etymology

ellipse + -oid

noun

1. (mathematics, geometry) A surface, all of whose cross sections are elliptic or circular (including the sphere), that generalises the ellipse and in Cartesian coordinates (x, y, z) is a quadric with equation x²/a² + y²/b² + z²/c² = 1.
   Polarizability can be imagined as a three-dimensional ellipsoid centred on the centre of the molecule, as shown in Figure 10.4. 2002, John Michael Hollas, Basic Atomic and Molecular Spectroscopy, page 133
   Because only ellipsoids of rotation have been adopted in practical geodesy and surveying and triaxial ellipsoids have been limited to theoretical studies, we will use the term ellipsoid for brevity to mean ellipsoid of rotation. 2004, Alfred Leick, GPS Satellite Surveying, 3rd edition, page 367
   As mentioned before, modern geodetic datums rely on the surfaces of geocentric ellipsoids to approximate the surface of the earth. 2010, Jan Van Sickle, Basic GIS Coordinates, 2nd edition, page 73
2. (geography) Such a surface used as a model of the shape of the earth.
   Here the geoid is thirty meters below the ellipsoid.

adj

1. Shaped like an ellipse; elliptical.
2. (mathematics) Of or pertaining to an ellipse; elliptic.
3. (botany) Having the tridimensional shape of an ellipse rotated on its long axis.