n :  the time required for half of something to undergo a process: as  a  :  the time required for half of the atoms of a radioactive substance to become disintegrated  b  :  the time required for half the amount of a substance (as a drug or radioactive tracer) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes <the serum is about 2 days -Jour. Amer. Med. Assoc.>  .
 
 
 
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