Datum

Datum is a set of parameters and control points used to accurately define the 3-dimensional shape of the earth into a planar coordinate system.

For marine applications, Datum (vertical) is a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of tidal phenomena and is based on a 19-year tide cycle (in the USA)- the datum is referenced to a fixed point typically known as a bench mark.