Europium is a rare earth element, number 63 on the periodic table.
In 2365, while Ambassador Lwaxana Troi was visiting the Enterprise, she was particularly after Captain Jean-Luc Picard and to avoid being alone with her, he invited Lieutenant Commander Data to come by and provide them with some "interesting facts." He explained that for most stars, the rare earth element europium is enriched relative to samarium and gadolinium. However in brown dwarf stars, the element europium is anomalously depleted. (TNG: "Manhunt")
Europium was named for the continent of Europe.