: a bitter alkaloid C8H10N4O2 found especially in coffee, tea, cacao, and kola nuts and used medicinally as a stimulant and diuretic
Examples of caffeine in a Sentence
Her doctor told her to avoid caffeine.
Recent Examples on the Web
Some may be concerned that the caffeine in green tea would have a diuretic effect, but that's not necessarily true.
—
Angelica Bottaro, Verywell Health, 25 Feb. 2026
The active ingredients tend to be pretty much the same—taurine, guarana, and, most saliently, caffeine—but the marketing is gentler, the flavors are fruitier, and the cans are cuter.
—Ellen Cushing, The Atlantic, 25 Feb. 2026
Fermented Bifida and Lactobacillus help keep the scalp balanced, while caffeine, biotin, panthenol, and B vitamins support stronger-looking roots over time.
—Christa Joanna Lee, Allure, 24 Feb. 2026
Cut back on the nightcap, and on afternoon caffeine; ensure your room is dark and quiet; and build in a consistent wind-down routine.
—Erica Sloan, SELF, 24 Feb. 2026
See All Example Sentences for caffeine
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Angelica Bottaro, Verywell Health, 25 Feb. 2026
The active ingredients tend to be pretty much the same—taurine, guarana, and, most saliently, caffeine—but the marketing is gentler, the flavors are fruitier, and the cans are cuter.
—Ellen Cushing, The Atlantic, 25 Feb. 2026
Fermented Bifida and Lactobacillus help keep the scalp balanced, while caffeine, biotin, panthenol, and B vitamins support stronger-looking roots over time.
—Christa Joanna Lee, Allure, 24 Feb. 2026
Cut back on the nightcap, and on afternoon caffeine; ensure your room is dark and quiet; and build in a consistent wind-down routine.
—Erica Sloan, SELF, 24 Feb. 2026
See All Example Sentences for caffeine
Cite this Entry
“Caffeine.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/caffeine. Accessed 4 Mar. 2026.
Copy CitationKids Definition
caffeine
noun
caf·feine
ka-ˈfēn 
ˈka-ˌfēn
: a bitter stimulating compound found especially in coffee, tea, cacao, and kola nuts
Medical Definition
caffeine
noun
caf·feine
ka-ˈfēn, ˈka-ˌ; ˈkaf-ē-ən
: a bitter alkaloid C8H10N4O2 found especially in coffee, tea, and kola nuts and used medicinally as a stimulant and diuretic
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