Magnesia n. : 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.

Example:
What happens if you mix Milk of Magnesia with phenolphthalein?

Taxonomy
Magnesia, prefecture, mountain, church, List of settlements in the Magnesia prefecture, municipality, Magnesia on the Maeander, temples, Pelion, Province, monasteries, capital, Greece, Larissa prefecture, free encyclopedia, Greek, peak, Almyros.

Defination from the web:
Magnesia prefecture is the metropolitan city of Volos one of the most scenic and developed urban areas in Greece. Magnesia is located only half way between Athens and Thessaloniki. ... In its interior Magnesia has two plains. ...