"Unix" implies paying annual fees for use of the "Unix" trademark, and/or at least direct descent from the original Unix code.

According to: https://kb.iu.edu/d/agat "To use the Unix trademark, an operating system vendor must pay a licensing fee and annual trademark royalties to The Open Group. Officially licensed Unix operating systems (and their vendors) include macOS (Apple), Solaris (Oracle), AIX (IBM), IRIX (SGI), and HP-UX (Hewlett-Packard). Operating systems that behave like Unix systems and provide similar utilities but do not conform to Unix specification or are not licensed by The Open Group are commonly known as Unix-like systems."

Many will include the *BSDs as a Unix, because their code does directly descend from the original Unix code. But Linux distros generally do not meet either definition of "Unix".

It should be noted that Linux generally does actually conform to UNIX standards, though, and that two actual Linux distributions (Inspur K-UX and EulerOS) have in the past obtained UNIX certification. While this doesn't make all Linux distributions UNIX certified, it puts a rather large dent in the claim that they cannot be qualified as UNIX because of some claimed divergence from the standards.

(It also seems odd from my perspective to call exactly only those two Linux distributions "UNIX" unless you're essentially using it as a legal qualification and not a practical one)

> While this doesn't make all Linux distributions UNIX certified, it puts a rather large dent in the claim that they cannot be qualified as UNIX because of some claimed divergence from the standards.

No one is claiming it can't be a Unix. But as you noted, Linux distributions normally do not meet the legal criteria, nor are they descended from one that did.

Legally Unix is a trademark and has a very specific legal meaning. If you don't mean that legal meaning, then it is clearer if you use another term. The usual term is "Unux-like"; that is the widely-used term and it has been for decades.

A rose by any other name may smell as sweet, but calling it a different word risks confusion.

Only if their code for the certification is fully available upstream.

Part of it is whether the code can be traced back to original AT&T code. Which would be true for e.g. BSD variants (which includes MacOS). https://i.redd.it/kgv4ckmz3zb51.jpg

Another part is the trademark and certification fee. https://www.opengroup.org/openbrand/Brandfees.htm

Darwin forked BSD, and BSD is a fork of the original Unix source. Linux is a fresh implementation of POSIX, and doesn't directly inherit any code from Unix.

Linux is as much Unix as WSL1 was Linux - i.e. not at all, just clones.