Halo designer, David Ellis, was quick to state that it actually wasn't called Halo 5, and that we should wait for an update from @HaloWaypoint to confirm the game's name.
Since then, both @HaloWaypoint and Halo community manager, BS Angel, have been referring to it as "Halo Xbox One" and a folder has been added to the Halo Waypoint forums titled "Halo Xbox One"
"Halo Xbox One" may be a working title, and judging by some people's revulsion, there is a large section of the Halo community who hope it is. One thing we know for sure though, is that it won't be called Halo 5.
We can infer quite a lot from this; everyone, including key Microsoft employees, thought it would be called Halo 5. So if they were sticking with numbered titles, there would be no need to be coy about it, they would have just come out and said "Bam! It's Halo 5! Go pre-order."
There are several reasons to step away from numbered sequels. In a heavily story focused franchise like Halo, it can be very daunting for new players, people would feel they need to play Halo 1-4 before playing 5, a particularly troublesome issue on Xbox One where there is no backwards compatibility. Dropping the number also removes a lot of expectancy people may have about feature parity with the previous games in the series. A game called "Halo 5" needs to be a first person shooter, starring Master Chief, with competitive multiplayer, co-op, theatre mode, forge etc. A game called "Halo Xbox One" can be whatever it wants, it could be a squad based third person shooter (It won't be, before you shout at me in the comments). There's also the issue that 343 may think dropping the number allows them to distance themselves from Bungie's Halo games, they may feel the need to step out of Bungie's shadow, and not having their game look like a direct continuation of Bungie's series is a good way to do that.
But could it really be called Halo Xbox One? Well, Nintendo have been naming their games after their consoles for decades (Super Mario Bros, Mario 64, Mario Kart Wii etc.), so it is by no means unprecedented. There's just something unsettling about it, Halo is such a huge, cross-media behemoth that tagging the console's name on the end just seems a little.. low rent? That said, I can certainly see the attraction; every Halo Xbox One advert becomes a double hit advert for the console as well and makes it crystal clear that this is a game you can't play anywhere else.
But what do you call the sequel? Halo Xbox One 2? Nintendo can name their games after their console because they only make each game once per console cycle, a strategy I'm sure Microsoft won't want to replicate. Do we end up in the muddy territory of subtitles? Halo Xbox One: Reclamation would surely be a few syllables past consideration.
They could, of course, just call it "Halo". There's an elegance and finality to it, along with all manner of implications: this is the definitive Halo, this is the best Halo, this is the one that matters. It does a lot of the marketing team's job for them.
As with so many things related to Halo, whatever title 343industries and Microsoft choose is sure to be controversial, there will be those who get upset and there will be those who feel it's the right decision. Naming a game is not something to be taken lightly, the message that the name sends out will be well considered and intentional.
They should totally call it:
"Halo: The Starhopping Misadventures of Master Chief and His Best Pal Tom"
- Cowboy Out.