1. fillers generally introduce new character that aren't in the original story
2. fillers also may introduce some new power not in the original story.
3. The filler plot proceeds.
4. The filler plot ends.
5. None of the characters or new powers are relevant outside of that specific time period.
6. You NEVER see them again, the powers are completely forgotten, and the whole thing is a "dream".
But there are some subplots of rather unimportant characters that I could consider "filler" yes, although that is usually depending upon the narrative, timing, and general relevance to the story.
For instance i'd consider a story of someone unimportant in the middle of a random event that last 1-2 episodes, and then doesn't become relevant to the plot later as that character is still unimportant filler.
But a story of an unimportant character to show the world-view(daily life), a plotpoint that couldn't be viewed otherwise, or the relation they have with another character in a meaningfull way that makes them more valuable to the story: not a filler.