The chemical element (symbol Mg) with an atomic number of 12. It is a light, easily flammable, silvery-white alkaline earth metal.

(inorganic chemistry) The base Mg(OH)₂ that occurs naturally as brucite; it is used as an antacid and in many industrial processes

(inorganic chemistry) The magnesium salt of hydrochloric acid, MgCl₂; it has many industrial applications including the electrolytic production of magnesium metal.

(inorganic chemistry) A white powder (MgO) with a high melting point; it occurs naturally as periclase, an ore of magnesium.

(chemistry) A fine white powder, MgO₂, used as a bleach and disinfectant

(physics) The major stable isotope of magnesium, ²⁴₁₂Mg, having twelve protons and twelve neutrons; it amounts to about 79% of the element in nature

(physics) A minor stable isotope of magnesium, ²⁶₁₂Mg, having twelve protons and fourteen neutrons; it amounts to about 11% of the element in nature

(physics) A minor stable isotope of magnesium, ²⁵₁₂Mg, having twelve protons and thirteen neutrons; it amounts to about 10% of the element in nature

(chemistry) The magnesium salt of lactic acid; sometimes used as a buffer of a food additive

The magnesium salt of sulfuric acid (chemical formula MgSO₄), commonly called Epsom salts.