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
The good news is that phthalates have a short half-life and leave the body within a few days, experts said.
—
Sandee Lamotte, CNN Money, 31 Mar. 2026
Caffeine has a half-life of about six hours, so a 2 PM coffee is still circulating at 8 PM.
—Allison Palmer, Charlotte Observer, 30 Mar. 2026
What Is Working Against You Caffeine has a half-life of about six hours, which means a 2 PM coffee still has half its caffeine circulating at 8 PM.
—Allison Palmer, Fort Worth Star-Telegram, 30 Mar. 2026
But if anger has a long half-life in this book, then so does love.
—Giles Harvey, New Yorker, 30 Mar. 2026
See All Example Sentences for half-life
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Sandee Lamotte, CNN Money, 31 Mar. 2026
Caffeine has a half-life of about six hours, so a 2 PM coffee is still circulating at 8 PM.
—Allison Palmer, Charlotte Observer, 30 Mar. 2026
What Is Working Against You Caffeine has a half-life of about six hours, which means a 2 PM coffee still has half its caffeine circulating at 8 PM.
—Allison Palmer, Fort Worth Star-Telegram, 30 Mar. 2026
But if anger has a long half-life in this book, then so does love.
—Giles Harvey, New Yorker, 30 Mar. 2026
See All Example Sentences for half-life
Cite this Entry
“Half-life.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/half-life. Accessed 1 Apr. 2026.
Copy CitationMedical 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
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