cesium

noun

ce·​si·​um ˈsē-zē-əm How to pronounce cesium (audio)
also -zhē-
: a metallic chemical element that is the most electropositive element known and that is used in photoelectric cells, in atomic clocks, and as a component of drilling fluid see Chemical Elements Table

Examples of cesium 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.
Those who fear they may have been exposed to elevated levels of cesium should contact their medical provider. — Mary Walrath-Holdridge, USA Today, 23 Dec. 2025 Within it, the team can vaporize mixtures containing uranium, cerium, and cesium, and then control how the vapor is cooled. — Christopher McFadden, Interesting Engineering, 24 May 2026 Cesium-137, or Cs-137, is a manmade radioisotope of cesium. — Sara Tenenbaum, CBS News, 22 Dec. 2025 Atomic clocks, including hydrogen masers and cesium beam clocks, rely on the natural resonant frequencies of atoms to tell time with extremely high accuracy. — Joe Hernandez, NPR, 21 Dec. 2025 See All Example Sentences for cesium

Word History

Etymology

New Latin, from Latin caesius bluish gray

First Known Use

1861, in the meaning defined above

Time Traveler
The first known use of cesium was in 1861

Rhymes for cesium

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Browse Nearby Words

Cesena
cesium
cesium 133
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Cite this Entry

“Cesium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/cesium. Accessed 7 Jul. 2026.

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Kids Definition

cesium

noun
ce·​si·​um ˈsē-zē-əm How to pronounce cesium (audio)
: a silver-white soft element used especially in photoelectric cells see element

Medical Definition

cesium

noun
ce·​si·​um
variants or chiefly British caesium
ˈsē-zē-əm How to pronounce cesium (audio)
: a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells
—symbol Cs see Chemical Elements Table

More from Merriam-Webster on cesium

Nglish: Translation of cesium for Spanish Speakers

Britannica.com: Encyclopedia article about cesium

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