What makes a dungeon? [closed]

I’m a game design student, currently writing an article about dungeons. I’ve done quite an amount of research about these, and found out there’s a lot of different definitions for them, some of which contradict themselves. So I’d like you to tell me what, in your opinion, makes a dungeon.

Which things are necessary to make a place a dungeon?

Which things will automatically make a place a non-dungeon?

Which things are making no difference and can be found both in dungeons and in non-dungeons?

Is the term dungeon about structure, theme, content, location, purpose, story or something else?

Well, I think you get the idea. Please try to elaborate as much as possible your answers, I’m not afraid of huge amounts of text. And don’t hesitate to post anything that comes to your mind. I’m searching for clues and inspiration, so reasonibly far-fetched and unpopular opinions are more than welcome 🙂