During the Microsoft planning and early development process, Windows 10 was codenamed "Threshold." That codename, like a number of recent Microsoft ones, came from Microsoft's Halo franchise.
Because the most recent version of Microsoft's Windows client was named Windows 8, many assumed the next release would be Windows 9. Many inside and outside the company used "Threshold" and "Windows 9" interchangeably when discussing the coming Windows release. There have been rumors that Microsoft might opt to christen Threshold as "Windows X," "Windows 365," just plain "Windows" or "Windows One."
But Microsoft went instead with Windows 10 because they wanted to signify that the coming Windows release would be the last "major" Windows update. Going forward, Microsoft is planning to make regular, smaller updates to the Windows 10 codebase, rather than pushing out new major updates years apart. Windows 10 will have a common codebase across multiple screen sizes, with the UI tailored to work on those devices.
Microsoft is working on a number of related products and services that are part of the "Threshold" wave. The name of the version of Threshold that Microsoft is building to run on both ARM-based tablets and ARM-based Windows Phones is still not known. Nor is the "official" name of what some inside call Windows Server Threshold. (The most recent release of the Windows Server operating system is Windows Server 2012 R2.)
Microsoft is expected to release a preview test build of Windows 10 sometime in the next few days. The company is expected to finalize Windows 10 by the spring of 2015, according to sources.