The reference to which some quantity is measured. Also called the origin of a coordinate system. For example, any stated tidal sea level is implicitly stated relative to a datum that has been fixed at some known value. There is usually some large rock or piece of concrete with an embedded metal rod that precisely defines some coordinate in a reference frame that is used to measure sea heights. All sea height measurements in tide tables are relative to this datum mark.
Datums are also defined for measuring quantities on topographic maps such as the positions of roads and heights of mountains.
Whenever you see a single valued measurement (such as a height, a distance, a longitude or a latitude) it will be relative to some datum. See Define Geographic Coordinate System Dialog.