The story
From Ancient Greek λανθάνω (lanthánō, “to escape notice”) + -um (“a chemical element”), because it had gone long undetected in mineral ores.
Etymology from Wiktionary, CC BY-SA 4.0.
lanthanum
Today it means: A chemical element (symbol La) with an atomic number of 57: a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. Full definition →