Datum \Da"tum\, n.; pl. Data. [L. See 2d Date.]

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.

Any writer, therefore, who . . . furnishes us with data sufficient to determine the time in which he wrote. --Priestley.

2. pl. (Math.) The quantities or relations which are assumed to be given in any problem.

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.

Source: Webster's Revised Unabridged Dictionary (1913)