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

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

Cesium-137, or Cs-137, is a manmade radioisotope of cesium. —Sara Tenenbaum, CBS News, 22 Dec. 2025

The atomic clock, which uses cesium atoms to measure the exact length of a second, is used for GPS satellite networks, data centers, laboratories, aerospace, telecommunications, power generation and other systems that require ultra-precise timekeeping. —Lauren Penington, Denver Post, 21 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

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