What does cesium mean?
Cesium is a chemical element with the symbol Cs and atomic number 55, belonging to the alkali metals. It is a soft, silvery-gold metal notable for being among the most reactive elements on Earth: it ignites spontaneously in air and reacts explosively with water. Unusually for a metal, cesium melts at around 28.5 °C, only slightly above room temperature. Its scientific fame rests chiefly on precision timekeeping — the frequency of radiation absorbed or emitted by cesium-133 atoms serves as the international definition of the second, making cesium atomic clocks the backbone of global time standards, GPS navigation, telecommunications networks, and the internet itself. Cesium compounds are also used in photoelectric cells, infrared lamps, and as dense drilling fluids in the oil industry. In American English it is spelled 'cesium', while British English traditionally favors 'caesium'.
nounCesium (symbol Cs, atomic number 55) is a soft, silvery-gold alkali metal that is highly reactive and melts just above room temperature. It is best known for its use in atomic clocks, where the frequency of its atoms defines the SI second.
- A chemical element (Cs), atomic number 55: a soft, gold-colored, extremely reactive alkali metal used in atomic clocks, photoelectric cells, and drilling fluids.
"The laboratory's atomic clock measures time by tracking the vibrations of cesium atoms with extraordinary precision."
"Cesium's extreme reactivity means it must be stored in sealed ampoules under an inert atmosphere."
"GPS satellites carry cesium clocks so their timing signals remain accurate to within billionths of a second."
As a chemical element, 'cesium' is an uncountable mass noun; no plural form is used except informally when referring to different isotopes ('the cesiums' is not standard).
"The sample contained traces of cesium."
Every second of your life — and every GPS fix on your phone — is measured by cesium: exactly 9,192,631,770 vibrations of a cesium-133 atom define one second.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cesium
Cesium was discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff, who identified new spectral lines while analyzing mineral water with a spectroscope. They named it from the Latin caesius, meaning 'bluish-grey' or 'sky-blue', after the distinctive blue lines in its emission spectrum. This makes it one of the first elements ever discovered through spectroscopy rather than chemical analysis. The Latin root caesius also gives us the rare English words 'caesious' and 'caeruleous', and connects etymologically to the color vocabulary shared across Romance languages.
How cesium is actually used
Spelled 'cesium' in the US and officially by IUPAC; 'caesium' remains common in British usage. The word is technical/scientific in register and rarely appears in everyday conversation. The adjective form 'cesic' exists but is rare.
Easily confused with cesium
Caesium is simply the British spelling of the same element; 'cesium' is the American spelling standardized by IUPAC alongside 'aluminum'.