I’m actually kind of ok with the I know it when I see it approach.  I can say that in games I’ve played that use scenes like White Wolf and Feng Shui there hasn’t ever been a question.  The loose definition, which I think came from the first time I encountered it in an RPG in Vampire 1e, was based on a change of environment or narrative.

Scene 1: You show up at the warehouse to follow up on a rumor.
Scene 2:  You are still at the Warehouse when the fight breaks.
Scene 3: You leave the warehouse, head back to your base to lick your wounds and figure out what to do next.