UK:*UK and possibly other pronunciationsUK and possibly other pronunciations/ˈsiːzɪəm/US:USA pronunciation: respellingUSA pronunciation: respelling(sēzē əm)

WordReference Random House Unabridged Dictionary of American English © 2020
cae•si•um  (sēzē əm),USA pronunciation n. [Chem.]
  1. Chemistrycesium.

Collins Concise English Dictionary © HarperCollins Publishers::
caesium, US cesium /ˈsiːzɪəm/ n
  1. a ductile silvery-white element of the alkali metal group that is the most electropositive metal. It occurs in pollucite and lepidolite and is used in photocells. The radioisotope caesium-137, with a half-life of 30.2 years, is used in radiotherapy. Symbol: Cs; atomic no: 55; atomic wt: 132.90543; valency: 1; relative density: 1.873; melting pt: 28.39±0.01°C; boiling pt: 671°C
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