Metal Gear Solid V, that's what's been revealed as The Phantom Pain and MGS: Ground Zeroes at a GDC panel taking place right now. Want proof?
Check out the trailer below.
As you can see Keifer Sutherland voices the character helping snake out of hospital. The game is somewhat of a prologue, although also ties in Metal Gear: Ground Zeroes later in the game. It's all running on the new Fox Engine too.
So to be clear, Ground Zeroes and The Phantom Pain are in fact Metal Gear Solid 5.