Ellipsoid
The planet isn't a perfect sphere but presents a bumpy aspect with numerous irregularities.The notion ahead to address this imperfect form is the geoid. The geoid makes it possible to simplify the shape of the Earth, but it is not always regular.
This one still does not smooth enough the irregularities of the Earth, the scientifics introduce ellipsoids:“To model this surface, a more regular geometric figure is used, the ellipsoid: it is a generally spherical volume showing a flatness to the pole and used as reference for the building of map projections.” The ellipsoids are tangent to the surface of the globe and have two parameters: a parameter a (equatorial axis) and a parameter b (axis of the poles). There is not one but several ellipsoids. Each country has put one that best represents its territory: Ellipsoid of Clarke 1986 (North America), Ellipsoid International (Europe), Clarke 1880 (France).
(sources Géoconfluences, Samuel Depraz)
See : Projection (System of)
: Difference between Geoid and Ellipsoid