Real Dictionary

didymium in American English
a rare metal, formerly considered an element but later found to be a mixture of rare-earth elements neodymium and praseodymium: symbol, Di: the name is still used for naming oxides and salts
noun: a mixture of neodymium and praseodymium, formerly thought to be an element

didymium in British English
noun: a mixture of the metallic rare earths neodymium and praseodymium, once thought to be an element