lanthanum Definition
lanthanumnoun 1. chem.(symbol La, atomic number 57) a silvery-white metallic (one of the lanthanide series) that ignites spontaneously in air, and is used in rocket propellants, electronic devices, alloys for lighter flints and as a catalyst for the cracking of petroleum.
Etymology: 19c: from Greek
lanthanein to escape notice, because it is hidden in rare minerals.