Source : Webster's Revised Unabridged Dictionary (1913)

Magnesia \Mag*ne"si*a\ (?; 277), n. [L. Magnesia, fem. of
   Magnesius of the country Magnesia, Gr. ? ? ? a magnet. Cf.
   {Magnet}.] (Chem.)
   A light earthy white substance, consisting of magnesium
   oxide, 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}.

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

Source : WordNet®

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