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. With metallic element ending -ium. The first metal discovered by the aid of a spectroscope.
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Definitions of caesium from WordNet
a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal;