half-life

noun

ˈhaf-ˌlīf How to pronounce half-life (audio)
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
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
For the average person, caffeine has a half-life of about five hours. Carley Millhone, Health, 4 Oct. 2025 Iodine radioisotopes have half-lives that span from a tenth of a second to 16 million years. Andrea Richard, The Conversation, 2 Oct. 2025 One of the most popular, and notorious, alprazolam—more commonly known as Xanax—has a half-life of only twelve hours. P.e. Moskowitz september 11, Literary Hub, 11 Sep. 2025 For instance, rubidium-83 has a half-life of 82 days, meaning half the atoms would release neutrinos over nearly three months. Neetika Walter, Interesting Engineering, 9 Sep. 2025 See All Example Sentences for half-life

Word History

First Known Use

1907, in the meaning defined at sense 1

Time Traveler
The first known use of half-life was in 1907

Browse Nearby Words

Cite this Entry

“Half-life.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/half-life. Accessed 9 Oct. 2025.

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 How to pronounce half-life (audio)
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 daysThe Journal of the American Medical Association
Last Updated: - Updated example sentences
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