Datum

datum - Defines the origin, orientation and scale of the coordinate system and ties it to the earth, ensuring that the abstract mathematical concept 'coordinate system' can be applied to the practical problem of describing positions of features on or near the earth's surface by means of coordinates.

[Category=Geospatial ]

Source: Open Geospatial Consortium, 10 June 2011 08:55:39, http://www.opengeospatial.org/ogc/glossary

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datum - [geodesy] The reference specifications of a measurement system, usually a system of coordinate positions on a surface (a horizontal datum) or heights above or below a surface (a vertical datum).

[Category=Geospatial ]

Source: esri, 19 March 2012 08:19:37, http://support.esri.com/en/knowledgebase/GISDictionary/term/abbreviation

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Datum - A geodetic reference system that specifies the size and shape of the earth, and the base point from which the latitude and longitude of all other points on the earth's surface are referenced.

[Category=Geospatial ]

Source: City of Toronto, 23 November 2013 08:36:26, http://www1.toronto.ca/wps/portal/contentonly?vgnextoid=7fdf38379bac0410VgnVCM10000071d60f89RCRD&vgnextfmt=default