• An unstable isotope of an element that decays or disintegrates spontaneously, emitting energy (radiation). (NTI)
  CDISC Terminology
  Clinical Data Interchange Standards Consortium (CDISC), 2021
• A version of a chemical element that has an unstable nucleus and emits radiation during its decay to a stable form. (online dictionary)
  NCI Thesaurus
  U.S. National Cancer Institute, 2021
• An unstable form of a chemical element that releases radiation as it breaks down and becomes more stable. Radioisotopes may occur in nature or be made in a laboratory. In medicine, they are used in imaging tests and in treatment.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• (radionuclide) Isotopes that decay and emit radiation; either naturally occurring elements or are produced by the action of neutrons, protons, deuterons, or alpha particles; index with a specific element when appropriate, e.g., C14 index with CARBON and RADIONUCLIDE.
  CRISP Thesaurus
  National Institutes of Health, 2006
• Isotopes that exhibit radioactivity and undergo radioactive decay. (From Grant & Hackh's Chemical Dictionary, 5th ed & McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2021
• (radionuclides) Short-lived radioactive chemicals that are used in nuclear imaging tests.
  Harvard Dictionary of Health Terms
  Harvard Medical Publishing, 2011