The most important characteristic of a nuclear reaction is that it involves a change in the nucleus of an atom. This means that the number of protons or neutrons in the nucleus can change, resulting in the formation of a new element or a new isotope of the same element.

Here's why this is so important:

* Energy Release: Nuclear reactions are often accompanied by a tremendous release of energy, far greater than chemical reactions. This energy comes from the binding energy of the nucleus, which is the energy that holds the protons and neutrons together.

* New Elements and Isotopes: Nuclear reactions can create new elements, either by splitting heavy nuclei (fission) or by combining lighter nuclei (fusion). They can also create different isotopes of the same element by changing the number of neutrons in the nucleus.

* Applications: Nuclear reactions have a wide range of applications, including:

* Power generation: Nuclear power plants use nuclear fission to generate electricity.

* Medicine: Radioactive isotopes produced by nuclear reactions are used in diagnostic imaging and cancer treatment.

* Research: Nuclear reactions are used in scientific research to study the structure of matter and the fundamental forces of nature.

While there are other important characteristics, such as the conservation of mass-energy and the involvement of strong nuclear forces, the fundamental change in the nucleus is the defining characteristic of a nuclear reaction.