The traditional definition of filler is quite simple. It's not just any anime-original material or stuff that deviates from the "main plot" (if there even is one). Real filler is what happens in long-running shounen series like Bleach & Naruto; it's made when the anime catches up to the manga and needs to do its own crap for a while until enough new manga story has been produced.
Also, Aria rocks my world. :3
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