Relative Atomic Mass – The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of Carbon-12
Relative Isotopic Mass – The mass of an atom of an isotope compared with one-twelfth of an atom of Carbon-12
Relative Molecular Mass – The weighted mean mass of a molecule of compared with one-twelfth of an atom of Carbon-12
Relative Formula Mass – The weighted mean mass of a formula unit compared with one-twelfth of the mass of an atom of Carbon-12
Chemistry. 