1. A former name for the chemical element now called promethium (atomic number 61).
Usage: archaic; chemistry; The name was proposed in the 1920s but was not officially adopted; promethium is the standard modern name.

Examples

• Illinium was suggested as a name for element 61 before the scientific community settled on promethium.
• Early chemistry texts occasionally reference illinium in discussions of element nomenclature history.
• The discovery of element 61 led to several proposed names, including illinium, which ultimately fell out of use.