This is a villain who sticks around for the entire series or at least the majority of it, overlapping any story arcs. Because of that, they usually have a big role in the Myth Arc. The villain doesn't have to stick around for the whole series, but they must be around for a significant portion of it.
Sometimes, an Overarching Villain will be behind an Arc Villain or Monster of the Week if that villain is working for/being manipulated by them. This can result in the story being Hijacked by Ganon if the Overarching Villain has been introduced previously.
Not to be confused with Big Bad. The Big Bad is the villain who directly causes the bad things that the heroes are in conflict with. This is merely a villain who has a role for the entire series or at least a significant portion. While the Overarching Villain is often the Big Bad of an entire Myth Arc, they could simply be a frequently recurring villain.