An arc is a self-contained stretch of story within a longer series: a run of chapters or episodes with its own setting, conflict, and resolution, inside the larger narrative. Arcs are the standard unit fans use to divide long series.
The arc is the working unit of long-form fandom. Fans divide series into arcs to discuss them, databases index arcs, and wiki timelines are organized arc by arc, because the arc is the largest piece of story a viewer can hold in one thought. A well-built arc has its own cast additions, setting, rising problem, climax, and aftermath, and it changes something permanent in the series it belongs to.
Arc naming is a fandom institution of its own. Some arcs carry official names from the author or the guides; others are named by fans after their place, villain, or event, and the fan name becomes standard through use. The same arc often spans dozens of episodes and chapters, and its boundaries can be argued, usually at the seams where a quiet aftermath chapter belongs to one side or the other.
This site files quiz scopes by arc for exactly these reasons. An arc label tells a reader precisely which stretch of story a quiz covers, and the arc list on a subject's canon sheet is the map every other scope statement uses.
In use"The tournament arc ends, the journey arc begins: fans navigate a long series by its arcs."