Black tea is a variety of tea that is more oxidized than the oolong, green, and white varieties. All four varieties are made from leaves of Camellia sinensis. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. Two principal varieties of the species are used, the small-leaved Chinese variety plant (C. sinensis subsp. sinensis), also used for green and white teas, and the large-leaved Assamese plant (C. sinensis subsp. assamica), which was traditionally only used for black tea, although in recent years some green has been produced.

In Chinese and Chinese influenced languages, black tea is known as "crimson tea" (紅茶, Mandarin Chinese hóngchá; Japanese kōcha; Korean hongcha), an accurate description of the colour of the liquid. The term black tea refers to the colour of the oxidized leaves. In Chinese, "black tea" is a commonly used classification for post-fermented teas, such as Pu-erh tea; in the Western world, "red tea" more commonly refers to rooibos, a South African tisane.

While green tea usually loses its flavor within a year, black tea retains its flavour 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.The tea originally imported to Europe was either green or semi-oxidized. Only in the 19th century did black tea surpass green in popularity.[citation needed] Although green tea has recently seen a revival due to its purported health benefits, black tea still accounts for over ninety percent of all tea sold in the West.