* They involve changes in the nucleus of an atom. This is the defining characteristic. Nuclear reactions deal with the protons and neutrons that make up the nucleus, not the electrons orbiting around it.

* They release or absorb enormous amounts of energy. This is due to the strong forces holding the nucleus together. The energy released in a nuclear reaction is much greater than that released in a chemical reaction.

* They can involve the transmutation of elements. This means one element can be transformed into another by changing the number of protons in its nucleus.

* They are often accompanied by the emission of particles and/or radiation. Common examples include alpha particles, beta particles, gamma rays, and neutrons.

* They are governed by specific conservation laws. These include conservation of mass-energy, conservation of charge, and conservation of momentum.

It's important to note that nuclear reactions are very different from chemical reactions. Chemical reactions involve the rearrangement of atoms and molecules, while nuclear reactions involve changes within the nucleus itself.