Definitions

1. noun

a soft, silvery-gold alkali metal, the most electropositive of all elements, used in atomic clocks and in the production of other elements, especially cesium-137, a radioactive isotope used in medicine and research.

“The laboratory was working with a sample of caesium to develop a new type of atomic clock.”

2. chemistry

the chemical element with the symbol Cs and atomic number 55.

“Cesium is a highly reactive alkali metal with a wide range of applications.”