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

It persisted much longer in the fish, with a half-life exceeding 90 days and a strong tendency to build up in tissues.

From Science Daily ● Mar. 21, 2026

So why, in this glum post-“Walking Dead” era, awaken our much-loved caveman from a good death to such a miserable half-life?

From Salon ● Feb. 1, 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

For six years, Sobukwe lived a kind of half-life on the island; he was a free man who was denied his liberty.

From "Long Walk to Freedom" by Nelson Mandela

"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

Strange quarks are so dubbed because they have half-lives longer than expected, but they're still not very stable, especially when compared to electrons.

From Salon ● Oct. 27, 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

Alvarez determined that K-capture occurs only in heavy atoms with many protons—that is, those with high atomic numbers—-and in isotopes with long half-lives.

From "Big Science" by Michael Hiltzik