What is a “Dungeon”?

So, my wife and I were talking with some other gamers at our local game store, Tyche’s Games, as you do.  The people we were talked mentioned that their campaign did not not have dungeon and then described what their campaign did have . . . which sounded just like a dungeon just under another name.

Which lead to a discussion among my wife and I about what defines a dungeon.  I think I know what defines a dungeon, in the classic Dungeons & Dragons sense, to me and which I will elaborate on in a moment.  But what does the term dungeon mean to you?  What images does the word conjure?  Is there anything that you immediate think of, for good or ill?