A unit of mass equal to one twelfth the mass of an atom of carbon-12 is known as:

The molecular weight of a substance is the ratio of the mass of one molecule of the substance to the mass of one twelfth the mass of an atom of carbon-12. Because molecular weight is a ratio, it is dimensionless. The dalton (Da) is a unit of mass equal to one twelfth the mass of an atom of carbon-12. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 1.