Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

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

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
  • n. In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
  • n. The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
  • n. The time it takes for an idea or a fashion to lose half of its influential power.

from the GNU version of the Collaborative International Dictionary of English

  • n. the time it takes for one-half of a substance decaying in a first-order reaction to be destroyed. For radioactive substances, it is the time required for one-half of the initial amount of the radioactive isotope to decay. The half-lifeis a measure of the rate of the reaction being observed. For processes that are true first-order processes, such as radioactive decay, the half-life is independent of the quantity of material present, and it is thus a constant. The time it takes for one-half the remaining quantity of a radioactive isotope to decay will be the same regardless of how far the decay process has advanced. Some chemical reactions are also first order, and may be characterized as having a half-life. However, for chemical reactions the half-life will depend upon temperature and in some cases other environmental conditions, whereas for radioactive isotopes the rate of decay is largely independent of the environment.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. 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)
  • n. 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)

Etymologies

half- +‎ life (Wiktionary)

Examples

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  • if i'm right this is a game title. my aunt plays it all the time it is kind of cool.

    May 28, 2009