The Hawker Siddeley HS-121 Trident is a British airliner produced by Hawker Siddeley. In 1957, de Havilland proposed its DH.121 trijet design to a British European Airways (BEA) request. By 1960, de Havilland had been acquired by Hawker Siddeley. The Trident's maiden flight happened on 9 January 1962, and it was introduced on 1 April 1964, two months after its main competitor, the Boeing 727. By the end of the programme in 1978, 117 Tridents had been produced. The Trident was withdrawn from service in 1995.

An airliner is a type of airplane for transporting passengers and air cargo. Such aircraft are most often operated by airlines. The modern and most common variant of the airliner is a long, tube shaped, and jet powered aircraft. The largest of them are wide-body jets which are also called twin-aisle because they generally have two separate aisles running from the front to the back of the passenger cabin. These are usually used for long-haul flights between airline hubs and major cities. A smaller, more common class of airliners is the narrow-body or single-aisle. These are generally used for short to medium-distance flights with fewer passengers than their wide-body counterparts.