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. see the periodic table of elements in the Reference Supplement
Webster’s New World College Dictionary, Fourth Edition, compiled by the
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Wortherkunft von '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