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]