The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Any writer, therefore, who . . . furnishes us with
            data sufficient to determine the time in which he
            wrote.                                --Priestley.
      [1913 Webster]

   2. a single piece of information; a fact; especially a piece
      of information obtained by observation or experiment; --
      used mostly in the plural.
      [PJC]

   3. pl. (Math.) The quantities or relations which are assumed
      to be given in any problem.
      [1913 Webster]

   4. (Surveying) a point, line, or level surface used as a
      reference in measuring elevations. --RHUD
      [PJC]

   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.
      [1913 Webster]