half-life (t 1/2)
The time it takes for half the original amount of a substance to disappear.
In the environment, the half-life is the time it takes for half the original
amount of a substance to disappear when it is changed to another chemical
by bacteria, fungi, sunlight, or other chemical processes. In the human
body, the half-life is the time it takes for half the original amount
of the substance to disappear, either by being changed to another substance
or by leaving the body. In the case of radioactive material, the half
life is the amount of time necessary for one half the initial number of
radioactive atoms to change or transform into another atom (that is normally
not radioactive). After two half lives, 25% of the original number of
radioactive atoms remain. Source: ATSDR Glossary of Terms
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