caesium
/ˈsiːzɪəm/( <美> cesium)
noun
mass noun
- the chemical element of atomic number 55, a soft, silvery, extremely reactive metal. It belongs to the alkali metal group and occurs as a trace element in some rocks and minerals. (Symbol: Cs).（化学元素）铯（符号：Cs）。
词源
mid 19th cent.: from Latin caesius 'greyish-blue' (because it has characteristic lines in the blue part of the spectrum).