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.
The Xbox One OS, Windows 8.x OS and Windows Phone 8 OS already share a common Windows NT core. As we've heard before, Microsoft is working to deliver a single app store across its myriad Windows platforms. Company officials also are laboring to make the developer toolset for all three of these platforms more similar.
But Threshold will add another level of commonality across Microsoft's various Windows-based platforms, sources said. With the Threshold wave, Microsoft plans to support the same core set of "high value activities" across platforms. These high-value activities include expression/documents (Office, and the coming "Remix" digital storytelling app, I'd think); decision making/task completion (Bing, I'd assume); IT management (Intune and Workplace Join, perhaps?) and "serious fun."
CEO Steve Ballmer mentioned this concept of high-value activities at back in July when he announced Microsoft's cross-company reorg to make the company more focused around its new "One Microsoft" mission.