Datum in Geo Science
Datums are fixed reference points for elevation on the actual topographic surface or on a geoid/ellipsoid, it is also used to define the size and shape of the earth.
A geodetic datum is a model of the earth used to calculate coordinates on a map, for example WGS 84, one of the most widely used datums around the world (WGS stands for World Geodetic System).
The datums are either horizontally or vertically oriented.
The horizontal datum is the one that is used in measuring a specific position on the earth's surface in coordinate systems such as latitude and longitude.
The vertical datum measures the elevations of specific points on the earth.