In writing a novel, I've noticed that there are different scenes. It's a compilation of different scenes. What truly signifies the start, and end of a scene. If they are in a room, is that one scene? What happens if they walk out of that room, into another room and it's all one flow. Is that all still the same scene?
Is there a hard definition for when one scene ends, and one scene begins?
A scene is defined by the meaning we ascribe to the events.
Think of real life and how you tell your friend of something that happened. You usually have a clear idea of which of your lifelong, second-by-second experiences belong to that event (and in the narration), and which don't. If you tell of a visit to the dentist, you wont usually start with how you went to bed the evening before, or describe all the cars you saw on your way.
Writing is the same. Whenever an event has been completed and there is a sense of closure, the scene ends. This usually falls together with people meeting at the beginning and separating at the end of a scene, or with arriving at a location and leaving it, but is does not have to. It can also be the emotions or thoughts of the protagonist or narrator that structure the novel. In extreme stream of consciousness novels the text might not even have separate scenes (and chapters).
One helpful guideline is to think of a scene as being continuous in time and place (and perhaps in POV). So if your characters are in one place and they go to another, and your narrative continues with them, that can be a single scene. But if your characters are in one place, and then you jump (discontinuously) to another time or place, the jump would start a new scene.
That said, this is just a guideline. Some writers make leaps of time or place within a paragraph. Others sometimes begin a new scene even when the time, place, and POV are continuous. Stephen King has been known to begin a new scene in the middle of a sentence (e.g. in a scene in Dreamcatcher as a character awakes from a dream).
But I find it a mostly helpful guide. When time, place, or POV jumps, start a new scene.
