1. any narrative or legend of heroic exploits.
2. a form of narrative in which the members or generations of a family or social group are chronicled in a long and leisurely narrative.
In Ars Magica terms, a saga is the ongoing story of a group of Mythic Europe's magic users, called Magi. The Magi are bound together in agreement of and live within a Covenant, joined by any number of companions (non-magical yet important characters) and grogs (background characters like guards, craftsmen and administrative staff). The saga can last many lifetimes, following the exploits of the covenant's inhabitants as they study, play, fight, live and die in the purposes of the Covenant.