The definition of the world's busiest airport has been specified by the Airports Council International in Geneva, Switzerland. The ACI defines and measures the following three types of airport traffic:
Passenger traffic: total passengers emplaned and deplaned, passengers in transit counted once
Cargo traffic: loaded and unloaded freight and mail in metric tonnes
Traffic movements: landings and take-offs of aircraft
In the late 1940s, Chicago Midway was the busiest airport in the United States by total aircraft operations – i.e., including every training aircraft practicing take-offs and landings. New York LaGuardia had the most airline operations and passengers until the early 1950s, when Chicago became the busiest airport in the United States by any criterion. Before World War II, Chicago Midway was the origin or destination of one in four U.S. airline flights, although a 1939 Official Aviation Guide shows more airline flights scheduled at Newark than at Chicago.
The world's busiest airport by traffic movements during the seven-day EAA AirVenture Oshkosh event. 25,000 traffic movements are handled in a week during the yearly event, and air traffic controllers are picked by a competitive selection process to handle this traffic. During the event, a banner reading "WORLD'S BUSIEST CONTROL TOWER" is hung from the control tower.
Gatwick has the world's busiest single-use runway. It does have two runways on site but the close proximity of the two and the length and lack of guidance for the second means that only one is used. It is London's second largest international airport and second busiest by total passenger traffic in the United Kingdom after Heathrow.