Basically, nowadays Unix (or more formal POSIX) is a standard for operating systems to follow. Most modern operating systems are either derived from Unix (modern MacOS and Solaris), inspired by it (Linux and open-source BSDs) or at least POSIX-compatible (Haiku). Windows is neither of this - being monopoly they basically don't need to follow a standard when they can force everyone to implement their solution.

So yeah, Unix is an OS and Windows doesn't count.