What does elemental magnesium mean on a supplement label?

It is the amount of magnesium itself, not the weight of the whole compound. The Supplement Facts panel lists the elemental magnesium.^[1] For example, 500 mg of magnesium oxide gives 300 mg of magnesium, and 1,000 mg gives 600 mg.^[1] Medicines list it the same way: one tablespoon of a milk of magnesia laxative gives 500 mg of magnesium as magnesium hydroxide.^[1]