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

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

"PAM is a truly fascinating enzyme because it's the only enzyme we have that's capable of a chemical process called amidation, which increases the half-life or the potency of biologically active peptides," Gloyn said.

From Science Daily • Apr. 12, 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

The rules of physics dictated that an element with a half-life of 2.3 days had to be highly unstable.

From "Big Science" by Michael Hiltzik