lanthanum

(uncountable) A chemical element (symbol La) with an atomic number of 57: a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air.
(countable) An atom of this element.

atomic number 57

From Ancient Greek λανθάνω (lanthánō, “to escape notice”) + -um (“a chemical element”), because it had gone long undetected in mineral ores.

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