Nuclear Binding Energy
Definition
The energy holding an atomic nucleus together — the ‘glue’ energy you would need to split it into separate protons and neutrons.
Unit: MeV (typically per nucleon)
Technical definition
The energy equivalent of the mass defect: B = Δm·c², where the nucleus weighs less than its separate nucleons. Per-nucleon binding energy peaks at iron-56 (~8.8 MeV) — which is why both fission of heavy nuclei and fusion of light nuclei release energy.
Example
Helium-4's binding energy (~28 MeV total) is why the Sun shines — fusing hydrogen into helium converts mass defect into the sunlight we feel.
Binding energy comes from mass that ‘disappears’ — the nucleus literally weighs less than its parts. It is not stored somewhere; the missing mass became the energy holding it together (E = mc² in action).