The combination of each leader (stepped and dart) and their subsequent return strokes is known collectively as a "stroke". All strokes that use the same channel constitute a single "flash".
A flash can be made up of a single stroke, or tens of strokes. (The highest number of strokes ever recorded in a single cloud-to-ground flash is 47!)
Notice, in the greatly slowed down animation, the top part of a lightning flash is not connected to anything physical in the cloud, as the cloud itself is not conductive.
What happens is the lightning channel branches out inside the cloud in a tree-like shape, and draws free electrons to it (the negative charge in the lower half of the thunderstorm).