- The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance, such as a drug in serum, whose quantity decreases exponentially with time. Cf.:half-time.- biologic half-life the time required for one-half of an amount of a substance to be lost through biologic processes.- effective half-life the time required for the body burden of an administered quantity of radioactivity to decrease by half through a combination of radioactive decay and biologic elimination.- physical half-life the time required for half the atoms of a radionuclide to undergo disintegration.
* * *a) 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 (Jour. Amer. Med. Assoc.)>
* * *(hafґlīf″) 1. the time required for the decay of half of a sample of particles of a radionuclide or elementary particle, equal to 0.693 divided by the decay constant; symbol t1/2 or T1/2. 2. half-time.
Medical dictionary. 2011.