The DICT Development Group

  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]

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