The fictional M*A*S*H unit in the television series is the 4077th Mobile Army Surgical Hospital.
The series follows doctors, nurses, and support staff stationed with the 4077th during the Korean War. Although the unit is fictional, its name reflects the real MASH system: Mobile Army Surgical Hospitals were forward medical facilities designed to treat wounded soldiers close to combat zones and reduce evacuation time.
M*A*S*H premiered on CBS in 1972 and ran for 11 seasons, ending in 1983. The show adapted characters and situations from the 1970 film, which in turn was based on Richard Hooker’s 1968 novel. Its fictional 4077th was loosely inspired by real Korean War medical units, including the 8055th MASH, but it was not a direct portrayal of that unit.
The number is often confused with the historical units that inspired the story. “4077th” is the canonical designation used by the television series, while “MASH” itself is an acronym for Mobile Army Surgical Hospital.