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.

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. Here is the current certification page for OS X 10.9 'Mavericks' as 'UNIX 03' certified: Īpple has submitted OS X for certification (and received it,) every version since 10.5. Many different - not at all compatible - OSes are certified as a UNIX. 'UNIX' is really just a trademarked name, applied by The Open Group, upon completion of a certification.

Macbook pro os x operating system logo