The study of the characteristics, behavior, and internal structures of the atomic nucleus and its interactions with other nuclei. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)

The study of those aspects of energy and matter in terms of elementary principles and laws. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)

The science concerned with problems of radiation protection relevant to reducing or preventing radiation exposure, and the effects of ionizing radiation on humans and their environment.

The entities of matter and energy, and the processes, principles, properties, and relationships describing their nature and interactions.