half-life

American  

[haf-lahyf] / ˈhæfˌlaɪf /

Or half life,

noun

half-lives plural

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.)

Other Word Forms

Noun Inflected Forms

• half-lives
  plural

Etymology

Origin of half-life

First recorded in 1905–10; half + life

Example Sentences

See Examples For:

half-life half-lives

U.K. officials fret, though, that the half-life of the regal glow is diminishing fast.

From The Wall Street Journal ● Apr. 27, 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

If an isotope is said to have a half-life of five years, you can expect roughly half of the atoms to have decayed in that amount of time.

From "Meltdown" by Deirdre Langeland

"I'm already starting to think of what we can do next in terms of measuring their half-lives, their masses and other properties."

From Science Daily ● Feb. 15, 2024

Classic-film cable and streaming services have tended to have short half-lives.

From Los Angeles Times ● Jun. 29, 2023

Using this information, the collaboration deduced the half-lives of the isotopes; the team has already reported on five previously unknown half-lives.

From Salon ● Nov. 26, 2022

“And things that start out in the economics literature have half-lives in the applied policy world that are longer than the time period during which they’re the frontier of the field.”

From New York Times ● Aug. 25, 2022

After eight half-lives, only 1/256 of the original radioactive carbon remains, which is too little to make a reliable measurement, so radiocarbon dating works only for objects up to forty thousand or so years old.

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