ce·​si·​um | \ ˈsē-zē-əm also -zhē- How to pronounce cesium (audio) \

Definition of cesium

: 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 But officials have been engaged in a massive cleanup since, removing or treating swaths of topsoil to remove radioactive cesium and prevent it from entering vegetation. Washington Post, "Radioactive sushi: Japan-South Korea spat extends to Olympic cuisine," 23 Aug. 2019 Tokyo has stringent limits on the amount of cesium allowed in food, setting a maximum of just one-twelfth the levels permitted in the United States or the European Union. Washington Post, "Radioactive sushi: Japan-South Korea spat extends to Olympic cuisine," 23 Aug. 2019 The cesium would have to be separately glassified or disposed of some other way. Ralph Vartabedian, latimes.com, "Nation’s most ambitious project to clean up nuclear weapons waste has stalled at Hanford," 4 June 2019 Cesium atomic clocks are precision devices that measure time using the frequency of atomic emissions from the element cesium, according to a website maintained by the U.S. Naval Observatory. Joe Dworetzky, latimes.com, "South El Monte man charged with illegally exporting cesium atomic clocks," 28 June 2019 That includes elements like radioactive cesium, which have been known to make people sick to the point of death across the globe. David Grossman, Popular Mechanics, "Climate Change Could Unleash Long-Frozen Radiation," 15 Apr. 2019 The second is affixed to the motion of the atoms of the element cesium. Brian Resnick, Vox, "The world is about to redefine the kilogram," 15 Nov. 2018 Radiation detectors and dime-sized disks of plutonium and cesium used to make sure the detectors were working properly. Rachel Becker, The Verge, "The US government has trouble keeping track of radioactive material," 17 July 2018 Missing were radiation detectors and small samples of plutonium and cesium used to calibrate them. Dan Goodin, Ars Technica, "Dangerous plutonium stolen from rental car in a hotel parking lot," 16 July 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'cesium.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of cesium

1861, in the meaning defined above

History and Etymology for cesium

New Latin, from Latin caesius bluish gray

Keep scrolling for more

Learn More about cesium

Time Traveler for cesium

Time Traveler

The first known use of cesium was in 1861

See more words from the same year

Statistics for cesium

Cite this Entry

“Cesium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/cesium. Accessed 22 Feb. 2020.

Keep scrolling for more

More Definitions for cesium


variants: or chiefly British caesium \ ˈsē-​zē-​əm How to pronounce caesium (audio) \

Medical Definition of cesium

: 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

Keep scrolling for more

More from Merriam-Webster on cesium

Spanish Central: Translation of cesium

Nglish: Translation of cesium for Spanish Speakers

Britannica.com: Encyclopedia article about cesium

Comments on cesium

What made you want to look up cesium? Please tell us where you read or heard it (including the quote, if possible).


See Definitions and Examples »

Get Word of the Day daily email!

Test Your Vocabulary

More Confusing Words—Quiz

  • cats on impossible timber
  • The magician ______ moved the selected card to the top of the deck.
Spell It

Can you spell these 10 commonly misspelled words?

Add Diction

Test Your Knowledge - and learn some interesting things along the way.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!