Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that belongs to the lanthanide series of elements. Synonyms for terbium include gadolinite, ytterbite, and greenockite. These synonyms derive from the minerals in which terbium was first discovered. Gadolinite was found in a Swedish quarry in 1794 and named after Johan Gadolin, who was the first to isolate terbium. Ytterbite was discovered in a Finnish quarry in 1839 and was named after the village of Ytterby. Greenockite was named after the Scottish town of Greenock. Terbium is used in various applications, including green phosphors in television tubes and fluorescent lamps, as well as in nuclear reactors.