Prior to September 2014, Windows 9 was supposed to be the official name for the next generation of Microsoft's 'Windows' operating system for PCs, which was first hinted at in December 2013. The project's codename was 'Threshold', named after a planet in Microsoft's Halo video game franchise. It was to follow on from Windows 8 and Windows 8.1, which launched on October 26th 2012 and October 17th 2013 respectively.
However, on September 14th 2014, Microsoft unveiled Windows 10 at a press conference as the next major release for the series. Nothing further was said on the company's decision so skip over 9, however it is speculated that Windows 10 might be last major Windows release, with the company shifting to a smaller, iterative update model similar to the mobile-based Android and iOS platforms. Microsoft’s Terry Myerson is reported to have said that the time that "based on the product that’s coming, and just how different our approach will be overall, it wouldn't be right" for the product to be called Windows 9.
For further information, please see the 'Windows 10' Releases.Com entry.