\Di*dym"i*um\, n. [NL., fr. Gr. ? twin.] (Chem.)
A rare metallic substance usually associated with the metal
cerium; -- hence its name. It was formerly supposed to be an
element, but has since been found to consist of two simpler
elementary substances, neodymium and praseodymium. See
{Neodymium}, and {Praseodymium}.