Datum

1. Something given or admitted; a fact or principle granted; that upon which an inference or an argument is based; -- used chiefly in the plural.
2. a single piece of information; a fact; especially a piece of information obtained by observation or experiment; -- used mostly in the plural.
3. (Surveying) a point, line, or level surface used as a reference in measuring elevations.
Datum line (Surv.),
the horizontal or base line, from which the heights of points are reckoned or measured, as in the plan of a railway, etc.