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.
Nuclear binding energy
The mass defect of a nucleus represents the amount of mass equivalent to the 
binding energy of the nucleus (E=mc), which is the difference between the mass of a nucleus and the sum of the individual masses of the nucleons of which it is composed.