caesium
noun [ U ] UK (US cesium) uk/ˈsiː.zi.əm/ us/ˈsiː.zi.əm/ (symbol Cs)
铯（金属元素，银白色，质软，化学性质活泼，用于制光电池）
a chemical element that is a soft, silver-white alkaline metal. Caesium reacts strongly with other chemicals and is used in photocells.