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

Otherwise, today’s nuclear energy renaissance will have an even shorter half-life than the last one.

From The Wall Street Journal • Jun. 29, 2026

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

From The Wall Street Journal • Apr. 27, 2026

Those close to Khong marvel at the long half-life of her short stories.

From Los Angeles Times • Apr. 13, 2026

At-211's short half-life also means it quickly loses its radioactivity, making it less toxic than longer-lived radiopharmaceuticals.

From Science Daily • Nov. 11, 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