datum

(plural datums) A geodetic reference frame. In surveying and geodesy, a datum is a set of reference points on the Earth's surface, and (often) an associated model of the shape of the Earth (reference ellipsoid) used to dene a geographic coordinate system. Horizontal datums are used to describe the location of a point on the Earth's surface, in latitude and longitude or other appropriate coordinates. Vertical datums are used to describe site elevations or depths.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)

Original Source: NGS Glossary