It's nearly impossible to give a concise answer to what is Unix these days. In the early days BSD was a distribution of changes to Unix, extra features and ports to new platforms. Eventually, someone decided to distribute a complete package of BSD (derived from Unix) called 386BSD. Things happened and NetBSD and FreeBSD were born. Then the AT&T lawsuit stopped them from continuing. Most (but not all) original Unix code was removed and rewritten by the BSD folks at FreeBSD and NetBSD. Parts of FreeBSD are true Unix in a source sense.
SysV Unix diverged as well.. there are many modern Unix variants derived from it including Solaris, SCO Unixware (formerly Microsoft Xenix), etc. There's also divergences like MACH with Mac OS X being based on NeXTSTEP/OpenSTEP. The bsd family tree included in FreeBSD shows most of the divergences but does not include all of them.(MirBSD, MidnightBSD, ...)
SCO claims to own UNIX but courts ruled Novell has it. Novell sold the trademark to the Open Group and they sell a compatibility test suite and certification. Mac OS X, AIX and other things some people don't consider Unix are certified. At one point Linux was certified using the Open Group suite by IBM. Yet, Linux is considered a clone of Unix.
This becomes a religion question from here. Is it the source, the certification, the history... you decide.