Thulium is a chemical element with the symbol Tm and atomic number 69. It belongs to the group of rare-earth elements and is a silvery-grey metal that is quite soft and ductile. Synonyms for the word "thulium" include rare-earth metal, soft metal, lanthanide, and metallic element. Thulium is a moderately toxic element and has few practical uses. However, it is used in small amounts in some medical applications and as a dopant in fiber optics. Thulium was discovered in 1879 by Swedish chemist Per Teodor Cleve and was named after the mythical land of Thule. Despite being relatively obscure, thulium remains an important element for research purposes and is essential in the quest for practical applications of rare-earth metals.