1. Plural of illinium; a hypothetical chemical element that was once proposed but never confirmed to exist.
Usage: historical; chemistry; The term is now obsolete; illinium was speculatively assigned to element 61 before promethium was officially recognized.

Examples

• Early 20th-century chemists debated the properties of illiniums before the periodic table was fully understood.
• The name illiniums reflected attempts to name undiscovered elements after their suspected locations of discovery.
• Modern chemistry textbooks no longer reference illiniums, as promethium is the accepted name for element 61.