half-life \half"-life`\ haumf"limacf`, 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

half-life
     n : 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
see also:
half life