The half-life (or half-value period) of a substance is the time required for that substance to reduce to a size half of its initial value.

The half-life is most commonly encountered when talking about radioactive decay. Radioactive elements have different isotopes that decay at different rates. As a result the half-life has to be given in terms of the particular isotope under discussion. Some isotopes have very short half-lives, for example oxygen-14 has a half-life of only 71 seconds, some are even shorter with values measured in millionths of a second not being uncommon. Other elements' isotopes can have a much longer half-life, thallium-232 has a half-life of 1.4 x 10 ¹⁰ years and carbon-14 has a half-life of 5,730 years. This latter figure is used as the basis of radiocarbon dating of once-living organisms. While living, an organism takes in an amount of carbon-14 at a relatively constant rate. Once the organism...