a rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Symbol: Cs; atomic weight: 132.905; atomic number: 55; specific gravity: 1.9 at 20°C; melts at 28.5°C.
Also, caesium.

1930–35; < Neo-Latin, special use of Latin caesium, neuter of caesius bluish-grey; see -ium

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World English Dictionary
caesium or (US) cesium (ˈsiːzɪəm)
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
cesium or (US) cesium

cesium (ˈsiːzɪəm)
the usual US spelling of caesium

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Word Origin & History

coined by Bunsen and Kirchhoff in 1860, from L. caesius "blue-gray," in reference to the two blue lines in its spectrum.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

cesium ce·si·um or cae·si·um (sē'zē-əm)
Symbol Cs
A soft ductile metal, liquid at room temperature, the most electropositive and alkaline of the elements, used in photoelectric cells. Atomic number 55; atomic weight 132.905; melting point 28.4°C; boiling point 671°C; specific gravity 1.87; valence 1.

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American Heritage
Science Dictionary
cesium   (sē'zē-əm)  Pronunciation Key 
Symbol Cs
A soft, ductile, silvery-white element of the alkali group. It is liquid at room temperature and is the most reactive of all metals. Cesium is used to make photoelectric cells, electron tubes, and atomic clocks. Atomic number 55; atomic weight 132.905; melting point 28.5°C; boiling point 690°C; specific gravity 1.87; valence 1. See Periodic Table.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Cesium and other radio materials will enter into our food system.
So far the radioactive contamination has been iodine and cesium with no reports
  of strontium or other non-volatile elements.
The rabbits carried strontium and cesium, which emit gamma rays, back out of
  the area in their digestive tracts.
Naturally occurring cesium is not radioactive and is referred to as stable
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