The definition of 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
The following airports make claims based on objective volume measures that are defined above (as per ACI):
- Most passengers annually (1998-present)
- Most aircraft movements annually (2015-present)
- Most international passengers annually (2014-present)
- Most cargo traffic by weight annually (2010-present)
- Most passengers annually in all city airports combined (2010-present)
- 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 Midway 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.
- As the home of FedEx Express, Memphis had the largest cargo operations worldwide from 1993 to 2009. It remains the busiest cargo airport in the United States and the Western Hemisphere.