To break a nucleus apart we must supply energy to overcome the nucleons' strong nuclear attraction. This amount of energy is known as the binding energy.

As energy is conserved, if a nucleus forms from separate particles it must release this same amount of energy. This is where the energy released in nuclear fusion comes from.

We can observe this difference in energy before and after fusion by a difference in mass; the nucleus is actually lighter than the total mass of its particles. When nucleons form a nucleus, some of their mass is converted into energy and released. The difference in mass is known as the mass defect and is related to the binding energy by E = mc2.