Search Result for "half-life": 
Wordnet 3.0

NOUN (1)

1. the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate);
[syn: half life, half-life]


The Collaborative International Dictionary of English v.0.48:

Half-life \Half"-life`\ (h[aum]f"l[imac]f`), 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. [PJC]
WordNet (r) 3.0 (2006):

half-life n 1: the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate) [syn: half life, half-life]