Means of determining the age of certain materials by reference to the relative abundances of the parent isotope (which is radioactive) and the daughter isotope (which may or may not be radioactive). If the decay constant (the half-life or disintegration rate of the parent isotope) and the concentration of the daughter isotope are known, it is possible to calculate an age. See also dating methods; radioactive decay; radiocarbon dating; radiometric dating.