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]