After the Reagents of California-AT&T settlement (1995?), AT&T sold "UNIX" (term, trademark, code, everything) to Novell. This was when Novell was out there acquiring everything they could get their hands on. Some notable purchases during this timeframe was DR DOS and Word Perfect.
Sun paid Novell something like $3Mil to retain the right to call their code "UNIX" until the end of time.
Novell gave the Trademark (and subsequently name ownership) to the X/Open standards committee.
Novell sold the UNIX source to SCO (which is 30% owned by Microsoft).
SCO still owns the UNIX code today. They had a new release a year or two ago. Sun hasn't paid much attention to them since they've moneyed their way into a lifetime "UNIX" license.
You can get your OS stamped as "UNIX" from X/Open by paying a truckload of money to them for their inspection. They basically verify that your syscalls are compat with v7 UNIX (think "old stuff") as well as a few other issues. Obviously, community source projects like linux, FreeBSD, OpenBSD, or NetBSD don't have the massive financial resources required to get X/Open to certify their OS.
The end result is that the owners of "UNIX" are not overly aggressive (of AT&T fame) about the mis-use of the term.
If you want a comprehensive chart of timelines of various UNIX and UNIX-like operating systems, check out: [perso.wanadoo.fr...]
I hope this clarifies.