A Datum is a means by which coordinates determined by any means may be related to a well-defined Reference Frame. The Reference Frame may be visualised as a 3-D Cartesian coordinate system consisting, as a minimum, of information concerning the origin of the axes, and the directions of two principal axes fixed to the earth. The Reference Frame may be globally applicable, such as WGS84 or ITRF, in which case it is "geocentric" (having its origin at the earth's centre of mass), or be locally applicable as in the case of traditional national geodetic frames such as the Australian Geodetic Datum. In any case, the Datum may be considered synonymous to the Reference Frame, or be restricted to refer to the set of coordinates of geodetic stations or benchmarks which provide the physical realisation of the Reference Frame. A satellite-defined Datum such as WGS84 may, in addition, be realised by the time-varying coordinates of the satellites themselves (the Ephemerides). Finally, the Datum may be defined only in the horizontal sense or for the vertical component. An example of a Horizontal Datum is a Reference Ellipsoid (located and oriented in such a way as to be compatible to the Reference Frame to which it is attached), upon which coordinate information is expressed in terms of Latitude and Longitude. (WGS84 has a Reference Ellipsoid associated with it.) A Vertical Datum may be defined by a local realisation of Mean Sea Level, or as height above the Reference Ellipsoid.

This definition is in context to GPS. See more contextual defintions for Datum.