A radiogenic nuclide is one that is produced by a process of radioactive decay.
Lead is perhaps the best example of a radiogenic substance, as it is produced from the radioactive decay of uranium and thorium. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. Other substances considered radiogenic are argon-40, formed from radioactive potassium, and nitrogen, which is formed by the decay of carbon-14.