mag·ne·sia / magˈnēzhə; -ˈnēshə/ • n. Chem. magnesium oxide, MgO. ∎  hydrated magnesium carbonate used as an antacid and laxative.

magnesia
A. †(alch.) mineral ingredient of the philosopher's stone XIV;

B. †(spec. black m.) manganese XVII;

C. (spec. †white m.) hydrated magnesium carbonate, used medicinally; (chem.) magnesium oxide XVIII. — medL. magnēsia — Gr. (hē) Magnēsía (líthos) ‘the Magnesian stone’, (1) loadstone, (2) stone with silvery sheen; the development of senses B and C is obscure.
Hence magnesium XIX.

magnesia Magnesium oxide (MgO), a white, neutral, stable powder formed when magnesium is burned in oxygen. It is used industrially in firebrick and medicinally in stomach powders. Magnesium carbonate, found as magnesite and also used as an antacid, is often called magnesia.