Heathrow Airport (IATA: LHR, ICAO: EGLL), also colloquially known as London Heathrow Airport and named London Airport until 1966, is the primary and largest international airport serving England and the United Kingdom. It is the largest of the six international airports in the London airport system (the others being Gatwick, Stansted, Luton, City and Southend).

The airport is owned and operated by Heathrow Airport Holdings. In 2024, Heathrow was the busiest airport in Europe, the fifth-busiest airport in the world by passenger traffic and the second-busiest airport in the world by international passenger traffic. Heathrow was the airport with the most international connections in the world in 2024.

Heathrow was founded as a small airfield in 1930 but was developed into a much larger airport after World War II. It lies 14 miles (23 kilometres) west of Central London on a site that covers 4.74 square miles (12.3 square kilometres). It was gradually expanded over 75 years and now has two parallel east–west runways, four operational passenger terminals and one cargo terminal. The airport is the primary hub for British Airways and Virgin Atlantic.

The airport was opened on 25 March 1946 as London Airport; it was renamed Heathrow Airport in the last week of September 1966, to avoid confusion with the other two airports which serve London, Gatwick and Stansted. The airport's Central Terminal Area (CTA) was designed by Sir Frederick Gibberd, who set out the terminals and central-area buildings, including the control tower and the multi-faith Chapel of St George's.

Heathrow Airport is used by over 89 airlines flying to 214 destinations in 84 countries. The airport is the primary hub for British Airways and Virgin Atlantic. It has four passenger terminals (numbered 2 to 5) and a cargo terminal. In 2021 Heathrow served 19.4 million passengers, of which 17 million were international and 2.4 million domestic. The busiest year ever recorded was 2019 when 80.9 million passengers travelled through the airport. Heathrow is the UK's largest port by value with a network of over 218 destinations worldwide. The busiest single destination in passenger numbers is New York, with over three million passengers flying between Heathrow and JFK Airport in 2021.

In the 1950s, Heathrow had six runways, arranged in three pairs at different angles in the shape of a hexagram with the permanent passenger terminal in the middle and the older terminal along the north edge of the field; two of its runways would always be within 30° of the wind direction. As the required length for runways has grown, Heathrow now has only two parallel runways running east–west. These are extended versions of the two east–west runways from the original hexagram. From the air, almost all of the original runways can still be seen, incorporated into the present system of taxiways.

North of the northern runway and the former taxiway and aprons, now the site of extensive car parks, is the entrance to the access tunnel and the site of Heathrow's unofficial "gate guardian". For many years the home of a 40% scale model of a British Airways Concorde, G-CONC; the site has been occupied by a model of an Emirates Airbus A380 since 2008. Heathrow Airport has Anglican, Catholic, Free Church, Hindu, Jewish, Muslim and Sikh chaplains. There is a multi-faith prayer room and counselling room in each terminal, in addition to St. George's Interdenominational Chapel in an underground vault adjacent to the old control tower, where Christian services take place. The chaplains organise and lead prayers at certain times in the prayer room.

The airport has its resident press corps, consisting of six photographers and one TV crew, serving all the major newspapers and television stations around the world.