^ Good points Vasquez, and it would be hard to argue against Descent, which is one of my favorites.
Atmosphere could literally be the physical environment that the film takes place in, or the atmospheric type tension of the characters.
I more think of the environment, and I like to identify with that, and the circumstances where the characters find themselves. Probably why I like the secluded woods type area in Friday the 13th, as I like the woods and have been in that type of environment vs something like Descent. Though...Aliens would be another one and obviously never been to space.
For me, The Innkeepers, while I loved that movie, didn't do it for me as much atmosphere wise. Guess the thought of a hotel in a downtown area isn't as scary. While something like Shining, Vacancy, and Psycho does have the element I'm looking for.
I mentioned Halloween 4 and Scream as great atmospheres. I think that achieved that in a great way as it took a city (albeit small) and gave it a deserted feeling as everyone was hiding from the threat. 28 Days Later as well...creepy.
Have you ever been driving around and it felt odd cuz no one was out...and you had that WTF type moment?