Lost Magic (失われた魔法 Rosuto Majikku) is a category of Magic that have been obliterated from the history of the world due to their immense power and the sheer gravity of their side effects. However, users of such Magic still exist, though the Magic itself is believed to be extremely rare. It is a form of magic that has long been forgotten and is very rare. Knowing a Lost Magic brings one closer to the "source of Magic".
This page is simply for reference.