Magnesium dichloride is a magnesium salt comprising of two chlorine atoms bound to a magnesium atom. It is a magnesium salt, an inorganic chloride and a magnesium halide.Magnesium chloride. An inorganic compound consisting of one magnesium and two chloride ions. The compound is used in medicine as a source of magnesium ions, which are essential for many cellular activities. It has also been used as a cathartic and in alloys.