What does caesium mean?
Caesium is a soft, silvery-gold alkali metal with atomic number 55 and the symbol Cs. It is one of the most reactive naturally occurring elements, igniting spontaneously in moist air and reacting explosively with water, so it must be stored carefully under inert conditions. Discovered spectroscopically in 1860 by Robert Bunsen and Gustav Kirchhoff, it takes its name from the Latin caesius, meaning bluish-grey, after the distinctive blue lines it showed in their spectroscope. Although found only in small quantities in minerals such as pollucite, caesium plays an outsized role in modern science: caesium-based atomic clocks are so stable that they define the international standard for the second, while the radioactive isotope caesium-137 is both a medical and industrial tool and a well-known component of nuclear fallout. British English spells the word caesium, while American usage prefers cesium.
nounA soft, silvery-gold chemical alkali metal, atomic number 55, that is highly reactive and used especially in atomic clocks and photoelectric cells.
- The chemical element of atomic number 55, a soft, highly reactive alkali metal (symbol Cs).
- In nuclear contexts: radioactive isotopes of caesium, notably caesium-137, produced by nuclear fission.
"The laboratory kept its supply of caesium sealed in glass ampoules under argon to prevent it reacting with air."
"Caesium's low ionisation energy makes it ideal for photoelectric cells that convert light into electrical current."
"Fallout from reactor accidents often contains caesium-137, a radioactive isotope with a half-life of about thirty years."
Rarely needed; the plural arises mainly when referring to multiple samples or isotopes of the element.
"The shipment contained several sealed vials of caesiums of varying purity."
The world's most precise definition of a second comes from caesium: exactly 9,192,631,770 vibrations of a caesium atom tick away one second.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of caesium
Caesium derives directly from the Latin adjective caesius, meaning bluish-grey or sky-blue. When Robert Bunsen and Gustav Kirchhoff identified the new element spectroscopically in 1860, they named it for the striking blue spectral lines that revealed its presence. It was one of the first elements to be discovered through spectroscopy, marking a milestone in analytical chemistry. The related English adjective caesious, describing a bluish-grey colour, shares the same Latin root.
Related word forms
How caesium is actually used
The spelling 'caesium' is standard in British English; American English typically uses 'cesium'. The word belongs to scientific register and appears almost exclusively in chemistry, physics, and technical writing rather than everyday conversation.