Carbon dating (carbon 14 dating, radiocarbon dating) is a scientific method of calculating the age of carbonaceous material, plant or animal material or objects made from such material that was at one time living. It is accomplished by measuring the amount of carbon 14 (C14 remaining in the material. Carbon 14 is a radioactive isotope that exists in very small amounts in all living animals and plants. While alive, Animalia, Plantae, Fungi, Protista, Archaea and Bacteria normally have about one carbon 14 atom for every trillion carbon atoms. At the moment of death the level of carbon 14 begins to diminish in a predictable and measurable way.
See Radiocarbon dating for more details.