Definitions for half-life-ˌlaɪvz
Random House Webster's College Dictionary
or half′ life`
the time required for one half the atoms of a given amount of a radioactive substance to decay.
the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
Origin of half-life:
half life, half-life(noun)
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)
The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
U.S. National Library of Medicine
The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity.
Dictionary of Military and Associated Terms
The time required for the activity of a given radioactive species to decrease to half of its initial value due to radioactive decay. The half-life is a characteristic property of each radioactive species and is independent of its amount or condition. The effective half-life of a given isotope is the time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological elimination.