magnesia \magne"sia\ ?; 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
see also:
magnet mgo magnesium magnesia alba