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]