Datum n. : Something given or admitted; a fact or principle granted; that upon which an inference or an argument is based; -- used chiefly in the plural.

Datum n. : The quantities or relations which are assumed to be given in any problem.

Example:
What does "Datum" mean when referring to Architecture?

Taxonomy
WGS 84, datums, Vertical datums, sea level, GPS, tide, coordinates, map, geodesy, reference point, elevations, surveying, geodetic datum, the North, reference frame, broadcast, orbits, gravity.

Defination from the web:
datum is a set of reference points on the earth's surface against which position measurements are made, and (often) ... Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a ...