A saga is a large division of a long story, bigger than an arc: a grouping of arcs forming one era or campaign of the narrative. Some franchises use saga as an official label; in general fandom usage it marks the largest scale of story organization.
Used loosely, saga is a fan's word for scale. A saga is the part of a story that feels like a whole era: many arcs, sometimes years of publication, gathered under one great movement, such as a war, a quest across a widening world, or the rise and fall of a generation. When a franchise names its own sagas, as some long-running series do in their guides, the word becomes official structure.
The distinction from arc is practical. An arc resolves; a saga accumulates. Arcs end with a battle or a departure, while a saga ends when the story's world is no longer what it was at the start, and the next saga begins on new ground. Retrospect often draws the saga lines, since only distance shows which arcs were one movement.
On this site, saga appears in quiz titles only where a subject's own canon sheet uses it, because the word means different things in different franchises. Elsewhere the arc, the named and countable unit, does the work. Box sets and streaming collections often adopt saga groupings for packaging, which shows how thoroughly the fan and official senses of the word have merged in practice.
In use"Three arcs about one war make the war saga: the arcs resolve battles, the saga resolves the war."