If we imagine a nucleus as forming from the coalescing of an appropriate collection of protons and neutrons, then energy will be released upon formation of that nucleus. The sum of the masses of the individual protons and neutrons will be greater than that of the nucleus. The mass loss is called the mass defect.

While it is possible to calculate the mass loss for each known nuclide, it is more interesting to determine the average mass loss per nucleon in the nucleus. This usually is represented as the binding energy per nucleon.

The Curve of Binding Energy. Alternate source: Nuclear Mass Defect