Nuclear Binding Energy
Definition
The energy that holds a nucleus together: equivalently, the missing mass when you weigh a nucleus versus its separate protons and neutrons.
Technical definition
Nuclear binding energy is the energy required to disassemble a nucleus into its constituent protons and neutrons, equal through E = mc^2 to the mass defect: the assembled nucleus weighs less than its parts. Binding energy per nucleon peaks at iron-56, explaining why both fission of heavy nuclei and fusion of light nuclei release energy.
Example
A helium-4 nucleus weighs about 0.7 percent less than 2 protons + 2 neutrons separately. That missing mass is the binding energy holding it together.
Binding energy per nucleon is the key number, not total binding energy. Iron-56's peak is why energy is released both by splitting uranium AND fusing hydrogen.