noun

1. a rare metal, formerly considered an element but later found to be a mixture of rare-earth elements neodymium and praseodymium: symbol, Di: the name is still used for naming oxides and salts
2. any commercial mixture of rare-earth elements found in monazite sand

Etymology: ModL: so named (1841) by C. G. Mosander (see lanthanum) < Gr didymos, twin (because assoc. with lanthanum) + -ium