Among the radionuclides of a decay chain, the state that prevails when the ratios between the activities of successive radionuclides remain constant.
Note:
This is not equilibrium in the strict sense, since radioactive decay is an irreversible process.
Source:
PAC, 2021, 93, 69. 'Vocabulary of radioanalytical methods (IUPAC Recommendations 2020)' on page 95 (https://doi.org/10.1515/pac-2019-0302)