I love this question, but have never found it particularly useful when asked generally without context. If there is no need to define juggling, all that is possible is circular definitions like "Juggling is what jugglers do".
So I have many different answers depending on the context. Two examples from competitions I've run:
When I organized the first BYJOTY show, I said the young jugglers could do anything they want on stage, as pretty much anything can be juggling, as long as some kind of prop was used. I also excluded yoyo, not because I wouldn't consider it juggling, but because yoyo already had its own competition scene. Same with unicycles, I think. Yoyo and unicycling were allowed, but only if combined with other props.
When writing up some strict rules about combat, I had to work out if someone was "still juggling" three clubs even if they were making a fumble or if their club was bouncing off another player. I ended up having to build up a new vocabulary with new concepts, and was left with passages like this:
"With these terms, and defining a catch as non-dynamic contact, we can define a valid three club pattern by stating: Non-dynamic contact with a specific club is allowed only when a player has a minimum of one other club in the air or in dynamic contact with the player's or opponent's body and/or clubs."
Similar very strict definitions are very useful when describing what is allowed in some kind of sports juggling competition like numbers juggling, combat or volleyclub.
But for juggling as an art form? Nope! Every juggler decides for themselves what is and isn't juggling, and usually that comes down to "what do I enjoy to do today?"