What is the X in X-mas?
lanthanide lan·tha·nide (lān'thə-nīd')
Any of the chemically related elements with atomic numbers 57 through 71. Also called rare earth, rare-earth element.
Any of a series of 15 naturally occurring metallic elements. The lanthanides include elements having atomic numbers 57 (lanthanum) through 71 (lutetium). They are grouped apart from the rest of the elements in the Periodic Table because they all behave in a similar way in chemical reactions. Also called rare-earth element. See Periodic Table.