A datum is the point or origin from which measurements, based on a specific model of the shape of the Earth, are taken to locate points, lines, and other geospatial features. A horizontal datum is used to map the positions (X and Y values) of points, and a separate vertical datum, which is a surface, is used to map the height (Z value) of points. Some names of commonly used datums are North American Datum 1983 (NAD 83) and World Geodetic System 1984 (WGS 84).
See FAQ 32318 "What's the difference among WGS 84, NAD 83, and NAD 27?" for more information on these specific datums.
