Also called a geodetic datum, a datum is a reference system to describe the ellipsoid, or Earth-shaped model used to define the origin and orientation of a coordinate system.  The most common datums used in Canada to translate coordinates to correct locations on the Earth are WGS84 (World Geodetic System 1984, based on the WGS84 ellipsoid) for geographic coordinates, and NAD83 (North American Datum 1983, based on the GRS80 (Geodetic Reference System 1980) ellipsoid for projected coordinates.