As you can see, a direct flight differs from a nonstop trip in that the latter travels nonstop while the former makes a brief stop en route to the destination (without any stopover).
Today, connecting flights or nonstop flights make up the majority of the available flights. However, back when aviation was just getting started, the fuel efficiency of aeroplanes was not as high as it is today. Therefore, in order for a long-distance aircraft to be able to fly the direct route, it was necessary for it to make at least one halt to refuel. And as technology developed over time, planes with no actual stops began to be known as nonstop flights, differentiating them.