In surveying and geodesy, a datum is a reference system or an approximation of the Earth's surface against which positional measurements are made for computing locations. A horizontal datum provides a reference frame for latitude and longitude coordinates on the Earth. It is a terminology that constitutes of a shape model and an anchor point.
WGS84 ellipsoid with an anchor point is WGS84 horizontal datum. When we apply WGS84 ellipsoid model to the earth, we get its geographic latitude (see figure below).
Local ellipsoids have been established to fit the Geoid (mean sea level) well over an area of local interest (figure below).
With increasing demands for global surveying, work is underway to develop global reference ellipsoids. In contrast to local ellipsoids, which apply only to a specific country or localized area of the Earth’s surface, global ellipsoids approximate the "geoid" as a mean Earth ellipsoid. More information on the "geoid" can be found here.