Elemental
The actual amount of a mineral in a compound, ignoring the weight of the carrier salt.
Minerals are sold bound to a carrier — a salt or an amino acid — so the milligram figure on the front of the bottle is usually the whole compound weight, not the mineral itself. "Elemental" refers to the actual mineral you get: elemental magnesium, elemental iron, elemental calcium, elemental zinc.
The elemental amount is listed in the supplement-facts panel and is the number to compare against the RDA and between products — a "1000 mg" label can deliver anywhere from a few dozen to several hundred milligrams of the actual mineral depending on the form.