Each release of Windows has a codename: the name the project is given internally, but not the name it will finally be released under. For Windows 8.1 in 2013, that name was "Windows Blue". For 2015, it appears we'll be talking about Windows Threshold.
Longtime Microsoft watcher Mary Jo Foley reports that Threshold is being used as the label for the 2015 release, and that the broad aim of that project is to create a more unified code base for Windows, Windows Phone and Xbox One. While all three of those platforms currently sport the Windows 'Modern' interface and have some common coding elements, developers still need to target each platform separately. That won't change completely with Threshold, Foley reports, but there will be more unified elements.
Threshold is a long way off, and it seems certain we'll see another update to Windows 8.1 before that, given Microsoft's commitment to a yearly upgrade cycle for its operating systems at both server and desktop level.