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**half-life:** The time taken for half of a given amount of a radioactive substance to decay into a non-radioactive substance. It is also defined as the time taken for half the quantity of a radioactive isotope in a sample to decay and form a stable element. It is the basis of radiocarbon and other radiometric dating methods. This decay rate, expressed as a statistical constant, is different for each isotope. If a sample, such as a piece of wood, has half of the original amount of radiocarbon remaining, then a time equivalent to the half-life has passed since it died. The half-life of radiocarbon is 5730 ? 40 years, while the half-life of radioactive potassium, used in potassium-argon dating is 1.3 billion years. The half-life in effect determines the general age range over which a radiometric dating method is potentially useful.