half-life

(hăfˈlīfˌ, häfˈ-)

1. Physics The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay.
2. Biology
   a. 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.