Valeri Vladimirovich Polyakov (Russian: , born Valeri Ivanovich Korshunov on 27 April 1942) is a Russian former cosmonaut. He is the holder of the record for the longest single stay in space, staying aboard the Mir space station for more than 14 months (437 days 18 hours) during one trip. His combined space experience is more than 22 months.Selected as a cosmonaut in 1972, Polyakov made his first flight into space aboard Soyuz TM-6 in 1988. He returned to Earth 240 days later aboard TM-7. Polyakov completed his second flight into space in 19941995, spending 437 days in space between launching on Soyuz TM-18 and landing on TM-20, setting the record for the longest time continuously spent in space by an individual in human history.

An astronaut (from the Ancient Greek ἄστρον (astron), meaning 'star', and ναύτης (nautes), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally reserved for professional space travelers, the term is sometimes applied to anyone who travels into space, including scientists, politicians, journalists, and tourists."Astronaut" technically applies to all human space travelers regardless of nationality or allegiance; however, astronauts fielded by Russia or the Soviet Union are typically known instead as cosmonauts (from the Russian "kosmos" (космос), meaning "space", also borrowed from Greek) in order to distinguish them from American or otherwise NATO-oriented space travellers. Comparatively recent developments in crewed spaceflight made by China have led to the rise of the term taikonaut (from the Mandarin "tàikōng" (太空), meaning "space"), although its use is somewhat informal and its origin is unclear. In China, the People's Liberation Army Astronaut Corps astronauts and their foreign counterparts are all officially called hángtiānyuán (航天员, meaning "heaven navigator" or literally "heaven-sailing staff").

Since 1961, 600 astronauts have flown in space. Until 2002, astronauts were sponsored and trained exclusively by governments, either by the military or by civilian space agencies. With the suborbital flight of the privately funded SpaceShipOne in 2004, a new category of astronaut was created: the commercial astronaut.