A big grey animal which features in the phrase elephant in the room
. Here it can mean anything you like that reminds you of an elephant in a confined space, such as:
1) A thing that's big, really big, and everybody is embarassed by the sight of it, so they pretend it isn't there: maybe it will go away.
2) Something so big you can think of nothing else but it.
3) A thing too big to fit, but after a while you get used to it, maybe even get to like it; it defines a scary new lifestyle.
All of these describe teenage obsessions, hence the title of Gus van Sant's teenage obsession movie Elephant.