half-life

noun

ˈhaf-ˌlīf 

ˈhäf-

1

: the time required for half of something to undergo a process: such as

a

: the time required for half of the atoms of a radioactive substance to become disintegrated

b

: the time required for half the amount of a substance (such as a drug, radioactive tracer, or pesticide) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes

2

: a period of usefulness or popularity preceding decline or obsolescence

slang usually has a short half-life

Examples of half-life in a Sentence

Recent Examples on the Web

But even materials with long half-lives can be hazardous over longer periods of time. —Julia Jacobo, ABC News, 21 June 2025

So, Lynch asks, what if dark matter has a half-life of roughly a billion years? —Lyndie Chiou, Scientific American, 30 Apr. 2025

Singapore's SkillsFuture provides citizens with credits for continuous education and retraining, acknowledging the shrinking half-life of technical skills. —Arafat Kabir, Forbes.com, 4 June 2025

Here’s what to know about caffeine’s half-life, plus the amount of caffeine found in your favorite coffee orders. —Jillian Kubala, Health, 2 June 2025

Word History

First Known Use

1907, in the meaning defined at sense 1

Rhymes for half-life

• alewife
• birdlife
• folklife
• housewife
• jackknife
• loosestrife
• midlife
• midwife
• nightlife
• penknife
• wildlife
• afterlife

Kids Definition

half-life

noun

-ˌlīf

: the time required for half of the atoms of a radioactive substance to change composition

Medical Definition

half-life

noun

ˈhaf-ˌlīf 

ˈhȧf-

: the time required for half of something to undergo a process: as

a

: the time required for half of the atoms of a radioactive substance to become disintegrated

b

: the time required for half the amount of a substance (as a drug or radioactive tracer) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes

the serum half-life is about 2 days

—The Journal of the American Medical Association