Cesium

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.
noun
the usual US spelling of caesium
n.

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.

cesium ce·si·um or cae·si·um (sē’zē-əm)
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; atomic weight 132.905; melting point 28.4°C; boiling point 671°C; specific gravity 1.87; valence 1.
cesium
(sē’zē-əm)
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.