What do you mean by nuclear energy? What is responsible for it's release?

In a nuclear change due to a radioactive phenomenon (such as decay, fission or fusion), the total sum of masses of product nuclei is always less than the total sum of the masses of reactant nuclei. Thus, there is a loss of mass, due to which the energy is released. The energy so obtained is called the nuclear energy.

The radioactive phenomenon responsible for the release of nuclear energy are fission and fusion.