Real Dictionary

radioactive series in American English
a series of unstable radioactive elements and isotopes, in which a given element or isotope beginning the series decays into and is succeeded by the next in the series, and so on until a stable nucleus is arrived at
noun: a succession of elements initiated in the radioactive decay of a parent, as thorium or uranium, each of which decays into the next until a stable element, usually lead, is produced

radioactive series in British English
noun: a series of nuclides each of which undergoes radioactive decay into the next member of the series, ending with a stable element, usually lead