Magnesium: A mineral involved in many processes in the body including nerve signaling, the building of healthy bones, and normal muscle contraction. About 350 enzymes are known to depend on magnesium. Magnesium is contained in all unprocessed foods. High concentrations of magnesium are contained in nuts, unmilled grains, dark-green leafy vegetables, legumes such as peas and beans, and fruit. Magnesium is thus readily available in foods that form the basis of a healthful diet. Magnesium deficiency can occur due to inadequate intake of magnesium, impaired intestinal absorption of the mineral, or excessive loss of it.