Nuclear data represents measured (or evaluated) probabilities of various physical interactions involving the nuclei of atoms. It is used to understand the nature of such interactions by providing the fundamental input to many models and simulations, such as fission and fusion reactor calculations, shielding and radiation protection calculations, criticality safety, nuclear weapons, nuclear physics research, medical radiotherapy, radioisotope therapy and diagnostics, particle accelerator design and operations, geological and environmental work, radioactive waste disposal calculations, and space travel calculations.

It groups all experimental data relevant for nuclear physics and nuclear applications. It includes a large number of physical quantities, like scattering and reaction cross sections (which are generally functions of energy and angle), nuclear structure and nuclear decay parameters, etc. It can involve neutrons, protons, deuterons, alpha particles, and virtually all nuclear isotopes which can be handled in a laboratory.

There are two major reasons to need high-quality nuclear data: theoretical model development of nuclear physics, and applications involving radiation and nuclear power. There is often an interplay between these two aspects, since applications often motivate research in particular theoretical fields, and theory can be used to predict quantities or phenomena which can lead to new or improved technological concepts.