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.

---

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 )

---

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.

---

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.

---

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

"Most pro-inflammatory mediators have a very short half-life, meaning they only last for minutes, not hours."

From Science Daily ● May 19, 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

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

“We ate at the restaurant. Pigeon says DeMarco’s pizza has a half-life of ten minutes. That’s from chemistry, and it means he only eats it fresh out of the oven.”

From "Liar & Spy" by Rebecca Stead

"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

Unlike the GLP-1 hormone, which is rapidly metabolized by the body, semaglutide and similar medications have much longer half-lives, meaning the medication can build up inside the body if not enough time elapses between doses.

From Los Angeles Times ● Dec. 20, 2023

And probably shorter half-lives than the sublimity past generations found in the technologies of their own day.

From Salon ● May 30, 2023

“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