A story arc is any stream of episodes that follow similar thematic lines and expressly or subtly reference/build upon one another. It doesn't matter if said episodes follow one after the other or if they're spread out over the course of a given season (or across numerous seasons).
Also, for the record, Memory Alpha's list of Voyager story arcs is actually incomplete, being that it is missing several additional story arcs such as Voyager's encounters with the Vidiians (Phage, Faces, Lifesigns, Deadlock, Resolutions), Voyager's encounters with the Borg (too many episodes to list here), Voyager's encounters with the Krenim (Before and After and Year of Hell), Voyager's encounters with Species 8472 (Scorpion 1 & 2 and In the Flesh), and Voyager's encounters with Captain Braxton (Future's End 1 and 2 and Relativity), just to name a few.