or cae·si·um

[ see-zee-uh m ]
/ ˈsi zi əm /


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.

Nearby words

  1. cesarean section,
  2. cesarevitch,
  3. cesarian,
  4. cesca chair,
  5. cesena,
  6. cesium 137,
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  8. cespitose,
  9. cess,
  10. cessation

Origin of cesium

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

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for cesium

British Dictionary definitions for cesium


/ (ˈsiːzɪəm) /


the usual US spelling of caesium
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for cesium



also caesium, rare alkaline metal, 1861, coined by Bunsen and Kirchhoff in 1860 in Modern Latin (caesium), from Latin caesius "blue-gray" (especially of eyes), in reference to the two prominent blue lines in its spectrum, by which it was first identified.

Online Etymology Dictionary, © 2010 Douglas Harper

Medicine definitions for cesium


n. 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.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Science definitions for cesium


[ sēzē-əm ]


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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.