Whew, tons of replies!

My own two cents: I really like how Fate Core defined it–a scene starts with one question and ends when that question is answered. Primetime Adventures does something similar. It’s nice, it’s clean, and it ties directly to moving the narrative forward. A scene exists to answer a question about the plot or about a character.

And, improv kinda has a similar idea: you call “scene!” when it feels like the main premise of the scene has played out. (Improv scenes are framed under very specific constraints, generally. This only really happens in improv games where you’ve got a small group doing a scene around a prompt. Usually two people.)