Elemental amount
For a mineral sold as a compound salt (e.g. magnesium oxide, zinc citrate), the mass of the actual mineral itself, excluding the weight of whatever it's bound to. A label declaring "1000mg magnesium oxide" is not the same as "1000mg of magnesium" -- oxide is only about 60% elemental magnesium by weight. Every dose comparison on this site normalizes to elemental amount so different salts of the same mineral are comparable.
Example
Magnesium oxide is 60.3% elemental magnesium by weight; magnesium glycinate is only 14.1% elemental magnesium, so a much larger total compound mass is needed for the same elemental dose.