noun a soft, silver-white, ductile, metallic chemical element, one of the alkali metals and the most electropositive of all the elements: it ignites in air, reacts vigorously with water, and is used in photoelectric cells: symbol, Cs; at. no., 55: a radioactive isotope (cesium-137) with a half-life of 30.17 years is a fission product and is used in cancer research, radiation therapy, etc.
Webster’s New World College Dictionary, Fourth Edition, compiled by the
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Word origin of 'cesium'ModL, orig. neut. of L caesius, bluish-gray (< IE base *(s)kai-, bright > -hood): so named (1860) by Bunsen because of the blue line seen in the spectroscope