half-life

American  

[haf-lahyf] / ˈhæfˌlaɪf /

Or half life,

noun

plural

half-lives

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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half-life British  

noun

1.  τ.  the time taken for half of the atoms in a radioactive material to undergo decay

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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half-life Scientific  

/ hăf′līf′ /

1. The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

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half-life Cultural  

1. 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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Discover More

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.)

Etymology

Origin of half-life

First recorded in 1905–10; half + life

Example Sentences

Enrofloxacin tended to stay dissolved in the water and was eliminated relatively quickly by lambari fish, with a half-life of about 21 days and low accumulation in tissues.

From Science Daily • Mar. 21, 2026

Deuterium is abundant, but tritium is scarce because it is radioactive, with a half-life of only 12.3 years.

From The Wall Street Journal • Dec. 28, 2025

Over the next year, the Snack Wrap disappeared— not vanished, exactly, but exiled to the Canadian menu, where it lived out a quiet half-life among hockey arenas and polite condiments.

From Salon • Jun. 18, 2025

His efforts to blame everyone else for his own failures are sure to have a very short half-life.

From Los Angeles Times • May 20, 2025

Imagine you had a sample of material with a half-life of 30 seconds.

From "A Short History of Nearly Everything" by Bill Bryson