1. a soft, silvery-white metallic element with atomic number 55, used in atomic clocks and photoelectric cells
Usage: also spelled caesium (British English); chemistry term

Examples

• Cesium is one of the most reactive metals and must be stored under mineral oil.
• The atomic clock uses cesium-133 to measure time with extreme precision.
• Cesium compounds are used in photoelectric cells and image sensors.
• Scientists discovered cesium in 1860 through spectroscopic analysis.
• The cesium fountain clock is the most accurate timekeeping device available.