Characteristic of certain atomic nuclei (radionuclides) to convert into other atomic nuclei without extraneous cause, emitting ionising radiation.

Measurand is "activity" i.e. the number of nuclear transformations of a radionuclide or radionuclide mixture occurring per time unit. The measurement is "Becquerel" (Bq), which corresponds to one nuclear transformation per second.

There are both radionuclides occurring in nature and artificial radionuclides produced by nuclear-physical processes. Characteristic for each radionuclide is its half-life.