radionuclide (Physics Dictionary)

radionuclide: Also called a radioactive nuclide, radioisotope, or radioactive isotope.

Any nuclide possessing excess nuclear energy to the point that it is unstable. Such excess energy is emitted through any of several processes of radioactive decay, resulting in a stable nuclide or sometimes another unstable radionuclide which can then undergo further decay. Certain radionuclides occur naturally; many others can be produced artificially in nuclear reactors, cyclotrons, particle accelerators, or radionuclide generators.