The chemical element (symbol Cs) with an atomic number of 55. It is a soft, gold-colored, highly reactive alkali metal.

(inorganic chemistry) The cesium salt of hydrochloric acid, CsCl

A radioactive isotope of caesium, ¹³⁷₅₅Cr, having 55 protons and 82 neutrons and which is formed mainly as a fission product by nuclear fission.

(obsolete) Francium.

A type of highly accurate atomic clock that is regulated by a resonance frequency of caesium atoms (the same frequency, 9,192,631,770 Hz, on which the precise scientific definition of the second is based).

(chemistry, inorganic chemistry) A yellow salt with the chemical formula CsAu, formed by the reaction of molten gold and molten caesium.