What is the definition of nuclear energy?

It's the energy produced by nuclear reactions.

Examples of famous nuclear reactions

Fission reaction

##quad_0^1 "n" + _92^235 "U" -> _56^141 "Ba" + _36^92 "Kr" + 3 _0^1 "n"##

Fusion reaction

##quad_1^2 "H" + _1^3 "H" -> _2^4 "He" + _0^1 "n"##Alpha decay

##quad_92^238 "U" -> _2^4 "He" + _90^234 "Th"##

Beta decay

##quad_6^14 "C" -> _7^14 "N" + "electron" + _0^0 barv##

We use fusion reactions to generate energy currently. In , we may be using fission reactions to generate energy.