The uranium-thorium decay series refers to a sequence of radioactive decay processes that begin with the isotopes uranium-238 or thorium-232. These isotopes undergo a series of transformations, emitting radiation and producing various daughter isotopes until they eventually stabilize into non-radioactive elements, such as lead. In the case of uranium-238, the decay series includes several intermediate isotopes, such as radium and radon, each with its own half-life. The entire process can take millions of years, illustrating the long-term nature of radioactive decay and its implications for geology and nuclear science.