Terbium is a lanthanide series, rare earth element denoted by the atomic symbol, Tb. Its atomic number is 65 and its atomic weight is 158.9254. It is a silvery-gray metal with a melting point of 2,473°F (1,356°C) and a boiling point of 5,846°F (3,230°C). It is malleable, ductile, and so soft it can be cut with a knife.

During the period from 1839 to 1843, Carl Gustaf Mosander attacked one of the most confusing and frustrating problems facing chemists of the time: the composition of the rare earth minerals. In 1839, he discovered that one of those minerals, erbia, contained at least two new elements, lanthanum and cerium.

In 1843, he attacked a second mineral, yttria, and found that it, too, was a complex mixture. This mineral eventually yielded ten new elements. Mosander is given credit for the discovery of one of these, terbium, although his work also contributed to the identification of others in the group. Terbium was isolated in a more pure form by Jean-Charles-Galissard de Marignac (1817-1894) in 1886.

Terbium, like erbium, yttrium, and ytterbium, are all named for the Swedish town of Ytterby, where the rare earth minerals were first found.

The double salt, TbF₃ZnS is used in phosphors in color television sets. When hit with n electron beam, the compound emits a green color. The element is also used as a dopant in calcium and strontium salts used in solid-state devices. Research shows that terbium may also be useful as a stabilizer in fuel cells.