Is Mac OS X UNIX?
All but one release of Mac OS X (now macOS) has been certified as Unix by The Open Group, starting with 10.5:
- 11.0 (Big Sur) on Intel Macs and on Apple Silicon Macs
- 10.15 (Catalina)
- 10.14 (Mojave)
- 10.13 (High Sierra)
- 10.12 (Sierra)
- 10.11 (El Capitan)
- 10.10 (Yosemite)
- 10.9 (Mavericks)
- 10.8 (Mountain Lion)
- 10.6 (Snow Leopard)
- 10.5 (Leopard)
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.
macOS'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.
Andrew Josey, VP Standards & Certification of the Open Group confirms that 10.7 Lion was never registered as a UNIX 03 product.
Well, given that it's fully POSIX compliant I would say yes.
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