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From The Collaborative International Dictionary of English v.0.48:
Didymium \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. [1913 Webster]