Didymium

n. [ NL., fr. Gr. &unr_; 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. [ 1913 Webster ]