It turns out the Microsoft codename for that wave of deliverables is "Threshold."
A couple of my contacts have confirmed that Microsoft Executive Vice President Terry Myerson recently mentioned the Threshold codename in an internal email about plans for his unified operating-system engineering group.
If all goes according to early plans, Threshold will include updates to all three OS platforms (Xbox One, Windows and Windows Phone) that will advance them in a way to share even more common elements.Microsoft Working on "Threshold" Update [zdnet.com]
From what I've heard, Threshold doesn't refer to a single Windows OS -- not even the expected, converged hybrid comprised of the Windows Phone OS and Windows RT. Instead, the codename refers to the wave of operating systems across Windows-based phones, devices and gaming consoles.