caesium

 
Pronunciation: /ˈsiːzɪəm/
(US 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)

Origin:

mid 19th century: from Latin caesius 'greyish-blue' (because it has characteristic lines in the blue part of the spectrum)