Black tea is a type of tea that is more oxidized than oolong, yellow, white and green teas. Black tea is generally stronger in flavor than other teas. All four types are made from leaves of the shrub (or small tree) Camellia sinensis. Two principal varieties of the species are used – the small-leaved Chinese variety plant (C. sinensis var. sinensis), used for most other types of teas, and the large-leaved Assamese plant (C. sinensis var. assamica), which was traditionally mainly used for black tea, although in recent years some green and white teas have been produced. In China, where black tea was developed, the beverage is called "red tea", due to the color of the oxidized leaves when processed appropriately.
While green tea usually loses its flavor within a year, black tea retains its flavor for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet and Siberia into the 19th century.