Medical Definition of half–life
: the time required for half of something to undergo a process: asa: the time required for half of the atoms of a radioactive substance to become disintegratedb: 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 half–life is about 2 days—Journal of the American Medical Association
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