Starshine: Operating Systems: Some Comments
Unix is one of the most widespread operating systems in the world. I think every major general purpose computing platform that's been introduced in the last decade or two as had some form of Unix available for it.
Despite this ubiquity there -- or due to it -- there is no single authoritative "Unix." There are numerous standards that apply. However, the current owner of the "coveted" Unix (tm) trademark is The Open Group (formerly X/Open and the OSF -- Open Software Foundation).
There are roughly 700 versions and implementations of Unix and Unix "clones." I get this number from the C-Kermit documentation -- since that's one of the most widely ported publicly available software pakcages. Linux 386BSD family: FreeBSD, NetBSD, and OpenBSD.