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 The stomach and tissue samples of the boars that researchers tested still continued to exhibit unsafe levels of cesium isotopes. Stephen C. George, Discover Magazine, 7 Nov. 2023 The National Institute of Standards and Technology keeps time with cesium fountain clocks, using lasers, vacuum chambers, clouds of ultracold atoms to nail the natural resonance frequency of the cesium—the basis for the standard second (9,192,631,770 Hz). WIRED, 27 Sep. 2023 In Germany, for example, scientists have long known that the wild boars roaming forests in the country’s south contain high levels of radioactive cesium. Will Sullivan, Smithsonian Magazine, 13 Sep. 2023 The research used a method involving the ratio of two cesium isotopes to analyze the carcasses of boars killed by hunters across Bavaria from 2019 to 2021. BostonGlobe.com, 2 Sep. 2023 In some samples, the cesium from weapons alone was enough to render the wild boars unsafe for human consumption. Tim Newcomb, Popular Mechanics, 1 Sep. 2023 The notoriously high levels of radioactive cesium found in European wild boars aren’t just from Chernobyl. Tim Newcomb, Popular Mechanics, 1 Sep. 2023 The researchers began by cooling individual cesium atoms down to almost absolute zero and get them all in the same quantum state. Jackie Appel, Popular Mechanics, 10 Aug. 2023 Time is defined in terms of cesium atoms, so the best clocks in the world are cesium clocks. Clara Moskowitz, Scientific American, 28 Jan. 2022 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'cesium.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


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

Dictionary Entries Near cesium

Cite this Entry

“Cesium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/cesium. Accessed 5 Dec. 2023.

Kids Definition


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


variants or chiefly British caesium
: 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

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