I've seen Deadpool and Deadpool 2, and they share a lot of similarities to spoof movies (Airplane, Scary Movie, Naked Gun). Granted this being a superhero movie with a lot of fun content — the scenes where Deadpool sees the X-Men in the school — makes me wonder if these movies really have any connection to the other films due to the content/rating, and would be more of a spoof/casual mention within the other films.
It is part of the X-Men Cinematic Universe.
Rhett Reese, the writer, has said in the following interview it is part of the X-Men universe which is essentially a short version for the full name the X-Men Cinematic Universe.
REESE: Yeah. Well, a lot of the bigger characters are spoken for, so when you go to get someone like Colossus, there are a lot of approvals and it has to fit into the larger X-Men universe and what the X-Men franchise is planning on doing with Colossus and who are you gonna cast and all those issues come in to play; it becomes a little bit of a quagmire sometimes, we did get approval for Colossus. But with Negasonic Teenage Warhead Tim actually gave us a pre-approved list, he’s like, “You can use any of these names” and we looked down the list and there were names like Bizarre, Tar Baby was a character, like, “I don’t think we’ll be using Tar Baby” But anyway, on that list it just popped, this name Negasonic Teenage Warhead.
REESE: And we are part of the X universe, and that is important. Simon Kinberg is our producer and he kind of minds the store in regards to the X universe, so we are fitting into the larger timeline, we need to recognize that. I mean, we have things like the Blackbirds in the movie, we reference Professor X in the movie. So Deadpool is very much of that world and I think sooner or later we’ll cross over into that world. I mean, we’re not sure when that would be. Probably, if I’d have to guess, we would do a standalone sequel before he entered the actual ensemble movies, but I think at some point it will cross over and it needs to fit.