In a comment to one of my question, a user mentioned that there is a difference between a direct and a non-stop flight. So what is this difference?
Although in everyday usage they are interchangeable, they refer to two entirely different concepts:
direct flight denotes any routing between two points with a single flight number, with one or more stops along the way. For example, United 803 flies IAD-BKK via NRT.
nonstop flight denotes air travel between two points with no scheduled intermediate stops. For example, United 645 is JFK-SFO with no stops.
In the past, a direct flight usually involved a single aircraft making multiple stops, in contrast to a connecting flight, in which you would leave one flight for another on a different aircraft. In the present, however, any such distinction is gone. United 951 ostensibly operates BRU-IAD-ORD, but you will not be on the same aircraft— there will be a "change of gauge" at IAD just as if it were an ordinary connecting flight.
Direct flights are tricky for frequent flyers. For one, most airlines compute your frequent flyer mileage based on distinct flight numbers, not on distinct flight segments. For flying BRU-IAD-ORD, you'd expect to be awarded 3893 miles for BRU-IAD and another 589 for IAD-ORD for a total of 4482; however, if you fly "direct" on UA951, you will receive only 4160, as if you had taken the nonstop. Another problem is in clearing upgrades. The BRU-IAD segment may have an upgrade available, whereas the IAD-ORD segment is sold out; however, you will not be given the upgrade because space must be available on the entire BRU-IAD-ORD itinerary. If you had flown the identical BRU-IAD-ORD itinerary but with two different flight numbers, you would have received the full mileage as well as the transatlantic upgrade.
Why do airlines continue to market direct flights? For the same reason they market codeshares: to make it seem as if they have service where they do not. Because such flights used to be on a single aircraft, the reservations systems weighted them as more preferable: request flights between two cities and they would return nonstops at the top, then direct, then connections. Since travel agents tended to pick flights that appear at the top of results, an airline offering a direct flight would likely get more sales than another which offered the identical itinerary as a connecting flight.
Here you go: Wikipedia to the rescue!
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.
Another source, from Matrix: (click the ? for their Glossary)
Flight/Direct flight = One or more legs, on the same airline, in which each leg has the same flight number
Non-stop flight = A flight with only one leg
A non-stop flight has no "other" stops, besides the point of destination.
A direct flight is sometimes known as a "continuing flight." That is, it will leave point A, stop at point B, and continue to point C. In a direct flight to point C, there is no danger of being left behind at point B (unless you get off the plane). Because if the plane arrives late at point B, it will also LEAVE late. But since you're on the plane, you'll get to point C.
This is in contrast to a "connecting flight," where you ride plane X from point A to point B, change to plane Y at point B, and fly from there to point C. Because if plane X arrives later at point B than plane Y departs, you will be "stranded" at point B.