Atomic mass
The atomic mass ("m") is the mass of an atom. Its unit is the unified atomic mass units ("abbr." u) where 1 unified atomic mass unit is defined as of the mass of a single carbon-12 atom, at rest. For atoms, the protons and neutrons of the nucleus account for nearly all of the total mass, and the atomic mass measured in u has nearly the same value as the mass number.
Atomic mass unit
The unified atomic mass unit and the dalton are different names for the same unit of measure. As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol, Da, is capitalized. With the introduction of the name "dalton", there has been a gradual change towards using that name in preference to the name, "unified atomic mass unit":