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half-life

or half life, half·life

[haf-lahyf, hahf-]
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noun, plural half-lives [haf-lahyvz, hahf-] /ˈhæfˌlaɪvz, ˈhɑf-/.
  1. Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
  2. Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
  3. Informal. a brief period during which something flourishes before dying out.
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Origin of half-life

First recorded in 1905–10; half + life
Also called half-life period.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for biological half-life

half-life

noun
  1. the time taken for half of the atoms in a radioactive material to undergo decaySymbol: τ
  2. the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for biological half-life

half-life

n.

also halflife, half life, 1864, with meaning "unsatisfactory way of living;" the sense in physics, "amount of time it takes half a given amount of radioactivity to decay" is first attested 1907.

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Online Etymology Dictionary, © 2010 Douglas Harper

biological half-life in Medicine

biological half-life

half-life


biological half-life in Science

half-life

[hăflīf′]

biological half-life in Culture

half-life

In physics, a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.

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Note

Scientists can estimate the age of an object, such as a rock, by carefully measuring the amounts of decayed and undecayed nuclei in the object. Comparing that to the half-life of the nuclei tells when they started to decay and, therefore, how old the object is. (See radioactive dating.)
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.