The Collaborative International Dictionary of English v.0.48:

Magnesia \Mag*ne"si*a\ (?; 277), n. [L. Magnesia, fem. of
   Magnesius of the country Magnesia, Gr. h` Magnhsi`a li`qos a
   magnet. Cf. Magnet.] (Chem.)
   A light earthy white substance, consisting of magnesium oxide
   (MgO), and obtained by heating magnesium hydrate or
   carbonate, or by burning magnesium. It has a slightly
   alkaline reaction, and is used in medicine as a mild antacid
   laxative. See Magnesium.
   [1913 Webster]

   Magnesia alba [L.] (Med. Chem.), a bulky white amorphous
      substance, consisting of a hydrous basic carbonate of
      magnesium, and used as a mild cathartic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

magnesia
    n 1: a white solid mineral that occurs naturally as periclase; a
         source of magnesium [syn: periclase, magnesia,
         magnesium oxide]