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
For example, the atomic mass unit (1 u) is defined as 1/12 of the mass of a C atom—but the atomic mass of a H atom (which is a proton plus electron) is 1.007825 "u", so each nucleon in C has lost, on average, about 0.8% of its mass in the form of binding energy.