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noun Half-life n. (Physics) The time it takes for one-half of a substance decaying in a first-order reaction to be destroyed. For radioactive substances, it is the time required for one-half of the initial amount of the radioactive isotope to decay. The half-lifeis a measure of the rate of the reaction being observed. For processes that are true first-order processes, such as radioactive decay, the half-life is independent of the quantity of material present, and it is thus a constant. The time it takes for one-half the remaining quantity of a radioactive isotope to decay will be the same regardless of how far the decay process has advanced. Some chemical reactions are also first order, and may be characterized as having a half-life. However, for chemical reactions the half-life will depend upon temperature and in some cases other environmental conditions, whereas for radioactive isotopes the rate of decay is largely independent of the environment.
Collaborative International Dictionary of English 0.48
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