didymium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A metallic mixture, once considered an element, composed of neodymium and praseodymium.
2. n. A mixture of rare-earth elements and oxides used chiefly in manufacturing and coloring various forms of glass.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, D or Di. A supposed element announced by Mosander in 1841, so named from being, as it were, the twin brother of lanthanum, previously discovered in the same minerals which yielded didymium, and from whose compounds those of didymium are separated with much difficulty. The most recent investigations indicate that didymium is not an element, but a mixture of several different elements.
2. n. [capitalized] A genus of fungi belonging to the Myxomycetes. The sporangia have a double wall, which is covered externally with crystals of lime, either scattered or compacted into a separable crust.

Wiktionary

1. n. A mixture of praseodymium and neodymium once thought to be an element (symbol Di).

GNU Webster's 1913

1. n. (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.