UNIX

UNIX is a family of preemptive multitasking, multiuser operating systems originating genetically from AT&T UNIX. UNIX was developed between 1969 to 1973 by Bell Labs, then a subsidiary of AT&T, and released, where it became a major force in the business, educational and research markets.

Variants

Three major genetic types of UNIX are recognized:

Research UNIX - The original 10 versions of UNIX developed by Bell Labs
Berkeley Software Distribution/Berkeley UNIX - Developed by UC Berkeley, BSD is a derivative of Research UNIX that is notable for several incidental innovations for TCP/IP.
System III/V UNIX - System III is a derivative of UNIX/32V, a research UNIX v7 derivative, which was later developed into System V, which had 5 major releases first under AT&T, and later under SCO/Xinuos.

UNIX and Silicon Graphics

All genetic siblings and parents of IRIX are offsprings of System V Release 3 or 4. GL2-2.x,RISC/OS, and 4D1 UNIX and early IRIX are System V Release 3, whereas IRIX 5.3 onwards is System V Release 4.

Confusion between Trademarks and Genetic UNIX

Since the 2000s, the Open Group, a foundation that justifies its existence through UNIX certifications, has aggressively pursued use of the UNIX trademark outside of its "Certified UNIX" which is, moreorless, conforming to the Single UNIX Specification and pay them hundreds of thousands of dollars. This absurd assertion of UNIX certification is evident with the certification of several GNU/Linux derived OSes as well as Apple macOS.

macOS uses Mach-O, which has never been used by UNIX as a file format, the mach kernel, a small amount of BSD kernel code and libc, but otherwise is either Mach-inspired or original in its design features.
GNU/Linux is two projects (a userland/libc and a system kernel) that are not specifically designed for interoperability. While GNU/Linux is closer conceptually to UNIX, it has deviated into its own APIs and conventions since the 2010s and is less and less UNIX-ish as time goes on. This is doubly so for the mobile OSes "Certified UNIX"

Hence, this is a racket by the Open Group. A racket as in "An easy and lucrative moneymaking enterprise, pejorative, to imply illegal or unethical practices." The Open Group sells trademark stickers, nothing more or less.