I had this argument recently saying Mac OS X was not UNIX, but Unix-like.
I know there is a Single Unix Specification and those spec compliant could use the UNIX trade mark.
Is Mac OS X a UNIX operating system or is it a Unix-like?
At any given time, Apple's page on The Open Group site only lists the current version of macOS and sometimes the previous version, but all of the links above were at one point found via that page.
OS X's status as a certified Unix is called out in Apple's Unix technology brief, which also has other good technical bits in it that will help you compare it to other UNIX® and Unix-like systems.
I monitor Apple's page on the Open Group's web site due to the popularity of this answer, and I never saw Lion show up there. Poking around with the clear URL scheme in the links above also turns up no hidden Lion certification page. Andrew Josey, VP Standards & Certification of the Open Group was asked if there is a missing certification, and he appears to have confirmed that 10.7 Lion was not registered as a UNIX 03 product.
Yes, OS X is UNIX.
"UNIX" is really just a trademarked name, applied by The Open Group, upon completion of a certification. Many different - not at all compatible - OSes are certified as a UNIX. OS X among them. Here is the current certification page for OS X 10.9 "Mavericks" as "UNIX 03" certified: http://www.opengroup.org/openbrand/register/brand3602.htm
Apple has submitted OS X for certification (and received it,) every version since 10.5. However, versions prior to 10.5 (as with many 'UNIX-like' OSes such as many distributions of Linux,) could probably have passed certification had they applied for it.
So it really depends on if you define "UNIX" as "the trademarked name by The Open Group, as applied to operating systems that have certification from The Open Group as a UNIX system" or if you define "unix" as "an operating system that functions like the original AT&T Unix operating system, and meets the standards set forward in any version of the Single Unix Specification, even if it was never submitted to The Open Group for testing and certification," then every OS X back to the original one would likely qualify. (As would most Linux distributions, even though none have undergone The Open Group certification.)
Oh, and I can't add a comment yet, but as an update to Warren Young's post - Apple did get UNIX certification for 10.7 (or at least they claim to have:) https://ssl.apple.com/media/us/osx/2012/docs/OSX_for_UNIX_Users_TB_July2011.pdf
One big difference is that X11 integration is a little different. X11 app will not looks good on a Mac, you have to manually start an XServer that's not given too much love. Other than that I think it's a Mach micro-kernel with the FreeBSD network stack, and the userland is like Linux.
Mac OS X 10.6 can compile the source code of the (already mentioned) book Advanced Programming in the UNIX® Environment, Second Edition if the
_DARWIN_C_SOURCE preprocessor flags are enabled (see compat(5) man page and here). So my answer would be 'Yes!'.
Can the source code of this book (somehow) be compiled on Windows as well?
(Re Linux-like userland: precompiled Mac OS X binaries of the GNU findutils package and other GNU tools are available at rudix.org).
In a legal sense yes. UNIX is a trade mark owned by The Open Group, which Apple has bought rights to use. Ditto for HP, IBM and Oracle regarding the UNIXes they sell.
However, unix (not capitalised) is also a specification which prior to 1986 was also copyrighted. It is no accident that commercial unix exploded in the late 80s and free unix followed along e.g. Linux in 1991.
Linux and FreeBSD no less than OS X (Darwin) technically qualify as unix but haven't paid the Open Group for UNIX certification and therefore cannot use the name.