Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

datum
     n : an item of factual information derived from measurement or
         research [syn: {data point}]
     [also: {data} (pl)]