noun.
A silvery metallic element, not a rare earth but occurring in nearly all rare-earth minerals, used in various metallurgical applications, notably to increase the strength of magnesium and aluminum alloys. Its oxide and other compounds are useful as phosphors, microwave filters, laser dopants, and superconducting materials. Atomic number 39; atomic weight 88.906; melting point 1,522°C; boiling point 3,345°C; specific gravity 4.469 (at 25°C); valence 3. cross-reference: Periodic Table.