Marathon is a term used to describe beatmaps that are five minutes in length or longer. They are often collaborative efforts, as mapping longer songs can be tiring when done alone.
When marathon beatmaps were first introduced, they were originally defined to be at least six minutes long, and could only include a single difficulty (with the exception that each beatmap could include a single osu!taiko difficulty as well).
Marathon beatmaps also had to have their single difficulty be named "Marathon" to bypass the Beatmap Submission System at the time and prevent the beatmap from entering the Pending category. Upon ranking, marathon beatmaps would enter the Approved category instead of the Ranked category to signal that these were marathon maps. The category was later abandoned in favour of the Ranked category.