The list of buildings
A city with an Airport produces veteran air units instead of regulars. An Airport is a link in the trade network, allowing luxuries and strategic resources to be traded to other airports.
Each airport and airfield can airlift one unit per turn, transporting it to another airport or airfield. The unit can't move again in that turn. Can't airlift armies.
Since the middle of the 20th century, the airplane has been one of the chief means of long-distance transport in the world. Nearly every major metropolitan area today has at least one airport, with facilities to handle passengers and ticketing, as well as facilities for refueling and repairing the aircraft that land there. Beginning in the 1940s, the growing role of airplanes as combat vehicles led to the widespread construction of military air bases. Nearly 400 of these facilities in the U.S. alone act as bases and maintenance facilities for aircraft assigned to all branches of the military.