What is nuclear energy?

It is the potential energy due to the strong nuclear force. Since the strong force is much stronger than gravity or electromagnetism, the amount of potential energy is enormous.

Nuclear energy may refer to:

Nuclear power, the use of sustained nuclear fission to generate heat and electricity

Nuclear binding energy, the energy required to split a nucleus of an atom

Nuclear potential energy, the potential energy of the particles inside an atomic nucleus

pick one, as each is a long discussion.