The term "direct flight" is not legally defined in the United States, but the Official Airline Guides define the term as a flight with a single flight number. While these flights may thus involve changes in aircraft, or even an airline at the intermediate point, they are typically differentiated from "connecting flights" in that the airline must enforce a dependency between multiple legs of the flight, so that leg two cannot operate if leg one has failed to arrive at the departure airport. Direct flights involving aircraft changes are also characterised by having planes at adjoining gates, instead of being located at random anywhere around the airport. Airlines enforce different policies on whether passengers may stay on the aircraft on routes which do not involve a change of aircraft.
Airlines may also market connections to a consolidation airport, usually a hub where the continuation of the flight from multiple aircraft is to a single aircraft listed under several flight numbers. Unlike traditional direct flights, multiple legs of such 'direct' flight actually operate as individual/independent legs, such that the latter leg can operate without any dependency or consideration of the former leg. In other words, the flight that comprises the latter leg can depart even if the flight that comprised the former leg failed to arrive.