Magnesium oxide is an inorganic compound with the chemical formula MgO. It is a white-colored hygroscopic solid. Magnesium oxide crystallizes in the form of a dense powder and is soluble in acid. It is an important raw material in the production of glass, phenolic, and plastic industries. It is also used in various refractory materials such as furnace linings and crucibles.

The IUPAC name of magnesium oxide is magnesia. It is produced by calcination of magnesium hydroxide or magnesium carbonate. It is a lewis base, which means that it can donate its nonbonding electron pairs to other substances. This property makes it an excellent catalyst for many organic reactions. It is highly reactive with water and can generate irritating or toxic fumes. It can burn skin and eyes when exposed to its fumes, and can irritate the respiratory system when inhaled.

Magnesium oxide has a high melting point and strong ionic bonding. It has a high thermal conductivity, but a low electrical conductivity value. Its crystal structure is a halite-like cubic cell with a space group Fm3m. In the crystal lattice, each magnesium ion (Mg2+) is surrounded by six oxygen ions (O2-), forming an octahedral coordination arrangement.

It can exist in two different forms, a bulky one called light magnesium oxide and a dense one called heavy magnesium oxide. The former is used for industrial applications, while the latter is used in medicine as an antacid and for preventing and treating low levels of magnesium in the blood.