When does a drop become a drip, a drip become a dribble, a dribble become a trickle, all the way up to a flow, a jet or a stream?

That is subject to the individual and collective judgement of observers. Until it is classified it is simply water moving through space, indifferent to your opinions. And yet, once subject to observation it both becomes something in the individual’s perspective and shifts the collective notion of just what it is. It’s identity and properties and description are filled in by the observer.