( BrE ) ( NAmE ces·ium ) / ˈsiːziəm; NAmE / noun
[ U ] ( symb Cs ) a chemical element. Caesium is a soft silver-white metal that reacts strongly in water, used in photoelectric cells .
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WORD ORIGIN
mid 19th cent.: from Latin caesius greyish-blue (because it has characteristic lines in the blue part of the spectrum).