cesium

noun

ce·​si·​um ˈsē-zē-əm 

 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

Far more dangerous than U-235 are the elemental products given off when nuclear fuel goes through fission—especially iodine, strontium, and cesium. —Jeffrey Kluger, Time, 24 June 2025

How Do Cesium Fountain Clocks Work? To achieve such exact timekeeping, NIST-F4 goes through a process involving cesium atoms. —Jack Knudson, Discover Magazine, 29 Apr. 2025

And make no mistake, even cesium atomic clocks are stunningly accurate. —IEEE Spectrum, 24 June 2021

To achieve very high precision, Luiten needed to find a material that could sustain electromagnetic oscillations for longer than a beam of cesium atoms can. —IEEE Spectrum, 24 June 2021

Word History

Etymology

New Latin, from Latin caesius bluish gray

First Known Use

1861, in the meaning defined above

Rhymes for cesium

• caesium
• magnesium
• trapezium

Kids Definition

cesium

noun

ce·​si·​um ˈsē-zē-əm 

: 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 

: 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