I'm pretty sure we were just discussing events?
It's a loose term, and it'd be odd for Frozen not to be considered one. Even with "only" 400m at the BO, it smashed in DVR, Album and Merchandise sales, as well as cultural relevance. The grand picture of the movie was unprecedented and lightning in a bottle.
Anyways, I still think anything over 400m is an event movie. Maybe 10 years ago or so that bar was set at 300m. It's an event because it gets a whole lot butts into soldout theaters, people make an event out of it, that's a near yearly thing.
Phenomenon is different. The Star Wars franchise itself is a phenomenon, Avatar and Titanic were phenomenons. Those are a few times a decade sort of thing.