The element that underwent radioactive decay to produce a radioactive isotope and eventually produce a more stable product. Can be by the emission of an alpha particle, two protons and two neutrons. or by the emission of a beta particle, emission of an electron which changes a neutron to a proton. Is used in the dating of objects because the decay happens in a predictable known pattern. [L. T. Aldrich. Measurement of Radioactive Ages of Rocks. Science, v123; 871-875; 1956.] [Analytical Chemistry. 69 :1692-99; 1997]