What defines a scene? Length, amount of descriptions, dialog?

Hm. Let me think about this for a while and get back to you. It's one of those things that I know, yet cannot quite define. Yet.

None of those.

Honestly, I'm not sure how well I could define a scene, but those aren't what I would use. Scenes can be any length, have any amount of description, and any amount of dialogue, including none.

Any one else able to define a scene?

A scene break is a break in narration. It's a passage of time, or a change of location. In a play it's a moment at which the stage is vacant because all of the current actors have left and none of the new ones have come on. It's a chapter within a chapter; small pieces that make up a whole. A chapter can be an entire scene; sometimes an entire book can be a single scene, or each scene can be a chapter, or you can break the chapters down into little scene separated by some sort of designated marker.

A scene(to me) is a setting where something important in a subplot or the overall plot happens. It should be described a little so that the reader knows where they are yet not too much to bore them.
When I read a book and then watch the movie, I find that they put less dialogue in the movie than there was in the book, and sometimes they manage to make sense. So maybe only put as much dialogue as is needed, unless you want to see character development within a subplot.
Sometimes there's action too, like a swordfight or struggle. Try not to draw it out too much but be sure to show the characters' emotion - fear, confidence, hate, sadness, surprise, etc. - through out each scene, because Character's emotion is important in an action scene so that it's not just like puppets moving around a stage.
And sometimes a scene is quiet and reflective of what's happened in the last scene while looking at a sunrise or sunset or some sort of beautiful scenery.
Hope that helps!

A scene is either the time spent in a certain setting or the full run of a lesser story arc.

To me, a scene is a span of "action" (narrative) with unity of place, time, and (in textual media) perspective. If you "cut away" from the "picture" and come back later, that's a new scene. If you follow the characters into a new location---what would, if this were a play or a screenplay, be a new "set"---that's a new scene. If you switch whose POV the story is being told in or filtered through, that had better be a new scene, and be marked with a typographical break (while the other two divisions can be softened with narrative transitions, that's a recipe for reader confusion), even though some authors don't do it that way.

Okay, I still can't explain it very well, so, let me try to show you.

Girl walks into bedroom. She sits down on the bed and un-braids her hair, then brushes it thoughtfully.

***
Girl is now in the kitchen, talking to her mother.


The first paragraph, before the asterisks, was a scene. Then, when time and setting changed, that marked the beginning of a new scene. See?

Darkness from Holy World's enemies is no match for the power of glitter.

What defines a scene? Torture evidently. ... glares* Where's the eye roll smiley when you need it.

A scene is a narrative unit in which events are described and play out close to "in real time" (as opposed to being glossed over, e.g. "I spent three weeks there hunting, fishing, and playing the banjo.") Whenever events are skipped or glossed over, ending the continuous action, a scene break is created. When a new sequence of continuous action begins, the same is a new scene.

That's my opinion, anyway.

Length doesn't really matter to me. What I look for in scenes in books is good descriptions and realistic dialog. Nothing too dramatic. I don't like it when books/movies make me think "Nobody says that in that situation!". Also, details. I like me some details. I love being able to picture every single thing going on. As if there was a TV in my brain!