So, saga is a majority portion or the overall story arc as a whole. An arc is just a portion of said story. I believe the term Saga came into play when DB came over here and Funimation started using the term for portions of the story. It's incorrect because just because it pertains to a certain villain doesn't make it a saga. The story as a whole is Goku's saga from start to finish. They're just story arcs in and amongst themselves.
