Caesium is mainly used in highly accurate atomic clocks (caesium‑133), which define the SI second. Radioactive caesium‑137 serves as a gamma source in radiotherapy, industrial gauges, and calibration. Nonradioactive compounds appear in oil and gas drilling (caesium formate brines) and in scientific research, electronics, and vacuum tubes. Some alternative medical uses lack reliable evidence and are not recommended.

Cs is the chemical symbol for element 55, a soft, silvery‑gold alkali metal. The name appears in two spellings: "caesium" (common in British English) and "cesium" (American English). Both refer to the same element and are accepted in scientific contexts; "Cs" is the universal chemical symbol used internationally. Pronunciation varies slightly by dialect.

Cesium’s value stems from its unique physical and chemical properties: the stable isotope cesium‑133 provides the precise atomic‑clock transition that defines the SI second, enabling GPS, telecommunications, and scientific timing. Its high density and low melting point make cesium compounds valuable in drilling fluids and industrial processes. Cesium’s photoemissive and ionization properties also support specialized electronics, research, and some medical/industrial radioactive applications.

Prices vary widely by form and purity. Elemental caesium typically costs thousands of dollars per kilogram (reagent-grade often hundreds per gram). Radioisotopes (e.g., caesium‑137) are tightly regulated and far more expensive and restricted. Over‑the‑counter cesium chloride supplements, though unproven and risky, generally sell for about $20–$100 per bottle. Exact prices depend on purity, quantity, and legal controls.

Cesium itself is not a dietary nutrient and offers no proven health benefits. Elemental cesium is highly reactive, and radioactive isotopes (like cesium‑137) are hazardous—causing radiation sickness, contamination, and increased cancer risk. Unproven therapies such as cesium chloride are potentially toxic and can cause serious side effects. Avoid unnecessary exposure and consult healthcare professionals for concerns.

Caesium is commonly considered the softest metal: a silvery-gold alkali metal so soft it can be cut with a knife and has a low melting point (28.5 °C), meaning it can liquefy near room temperature. Extremely reactive with water and air, caesium must be stored under inert conditions. Francium is heavier and unstable; its properties are largely theoretical and not practically observed.