A radionuclide is a radioactive nuclear species or nuclide. Of the 1,700 or so known nuclides, about 1,400 are radioactive. These radionuclides try to achieve a more stable structure by the emission of one or more different kinds of particles, including alpha particles. beta particles, and gamma rays, in the process known generally as radioactivity, or radioactive decay.

The activity of a radionuclide is the number of nuclei decaying per second. The standard unit is the becquerel, which is one decay per second. It is a very small unit and in general the terms kilobecquerel (kBq) and megabecquerel (MBq) are used.