1.Physics The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay. 2.Biologya. The time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes. Also called biological half-life. b. The time required for the radioactivity of material taken in by a living organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay.
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