1. A mixture of rare-earth elements, formerly thought to be a single element, consisting mainly of praseodymium and neodymium.
Usage: chemistry; historical; now largely obsolete as a term; the individual elements are used instead
2. A glass or lens containing didymium, used in welding goggles and scientific instruments to filter infrared and ultraviolet light.
Usage: technical; practical application

Examples

• Didymium was once classified as a single element before modern spectroscopy revealed its composite nature.
• Early chemists isolated didymium from rare-earth mineral deposits.
• Welders often use didymium lenses to protect their eyes from intense light and heat.
• The discovery of didymium contributed to the understanding of the periodic table.
• Didymium glass filters are still used in some specialized optical applications today.