1An aircraft, especially one which transports a large numbers of passengers on a frequently travelled route.
2 historical (The name of) a proposed public transit system in Pittsburgh designed in the 1960s, which used electric driverless trains on an elevated concrete track.
Known officially as the Transit Expressway Revenue Line (TERL). A demonstration track was unveiled in 1965 but the project met with political opposition and was cancelled in 1976.
Early 20th century; earliest use found in The Saturday Evening Post. From sky + bus.