What terminology describes the decay of a radioactive substance?

The term that most accurately describes the decay of a radioactive substance is "radioactive decay." This process refers to the spontaneous transformation of unstable atomic nuclei into more stable forms, which often involves the emission of radiation in the form of particles or electromagnetic waves. Radioactive decay occurs at a characteristic rate for each isotope, defined by its half-life, which is the time it takes for half of the radioactive atoms in a sample to decay.

In contrast, the other terms have specific meanings that do not capture the same concept. "Radioactive change" is a more general phrase and lacks the technical specificity used in the field of radiation safety. "Nuclear fission" refers specifically to the splitting of a heavy nucleus into lighter nuclei, which releases energy but does not encompass the broader concept of decay for various isotopes. "Radiolysis" involves the chemical decomposition of materials caused by radiation, usually in the context of interactions with water and other substances, rather than the decay of radioactive isotopes themselves. Therefore, "radioactive decay" is the precise terminology relevant to the process described in the question.