A direct flight in the aviation industry is any flight between two points by an airline with no change in flight numbers, which may include a stop over at an intermediate point. The stop over may either be to get new passengers (or allow some to disembark) or a mere technical stop over (i.e. for refuelling purposes only). These are often confused with non-stop flights, which are direct flights involving no intermediate stops. When there is a change in flight number, the subsequent flight is referred to as a connecting flight.
The term "direct flight" is not legally defined in the United States, but since the 1970s the Official Airline Guides have defined the term simply as a flight(s) with a single flight number. (In earlier years "direct" in the OAG did mean "no plane change".) While so-called "direct" flights may thus involve changes in aircraft, or even an airline at the intermediate point, they are typically -- but not always -- differentiated from "connecting flights" in that the airline will 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 or nearby gates, instead of being located at random anywhere around the airport. 
Airlines, airports, and security authorities in a particular country enforce different policies on whether passengers may stay on the aircraft on routes which do not involve a change of aircraft. For example, flights that require stopover merely for refuelling usually do not permit passengers to disembark from the aircraft. On the other hand, flights where there is a change of passengers may require that passengers disembark and stay at a holding area for security reasons and a proper headcount. In relation to this, a direct flight with stopovers may or may not involve a change in the flight crew.
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.