What is "railair"

RailAir describes a number of airport bus and coach services designed to connect the National Rail network to airports in the United Kingdom. Services are currently concentrated on London Heathrow Airport, with one other from London Luton Airport. RailAir services are operated as public transport services by or on behalf of train operators, where the whole journey is paid for as a through ticket which combines the railway and bus journey, although journeys can be made using the bus only. As such, many are operated where the train and bus operator are owned by the same company.