For a radionuclide
: the probability
in unit time for the decay of one of its nuclei by one of several modes of decay.
PAC, 1982, 54, 1533
(Glossary of terms used in nuclear analytical chemistry (Provisional))
on page 1547
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook