It's not really Unix. But it is a true descendant of Unix, unlike Linux. The original BSD was a branch of the Unix code. AT&T sued Berkeley though, so the final release of BSD, 4.4 which FreeBSD is based on, had no Unix code within it. FreeBSD evolved from BSD 4.4, and now it probably has little if any of the original code. So FreeBSD has came a long way since Unix. But the only other operating system that is a second descendant from it (if I'm not mistaken) is Solaris. So it is pretty much as close to Unix as you can get nowadays.