A soft, silvery-gray metallic element of the lanthanide series, extracted chiefly from the mineral monazite, used in high-temperature fuel cells as a stabilizer, in alloys responsive to a magnetic field, and in x-ray machines and lasers. Atomic number 65; atomic weight 158.925; melting point 1,356°C; boiling point 3,230°C; specific gravity 8.230; valence 3, 4. See Periodic Table.