To me filler only truly exists in TV shows.

Because a proper movie shouldn't have any filler because they shouldn't have enough time to be showing something completely separate from the story being told and only relating to the story indirectly.

But filler most commonly seen in animes is anything not directly related to the story that they tell over the course of an episode or more.

So basically if they take an entire episode or more to tell a side character or a non main character's backstory then that is filler

If they tell the back story within the episode while telling the main story then that is not filler.