*nix
*nix refers broadly to systems that are similar and possibly descendant from the Unix system and its general setup.
Actual UNIX is a historic thing from the seventies that led to variants starting in that same decade, commercial and academic.
(think SunOS/Solaris, HP-UX, IRIX, AIX, Minix, Ultrix, and Xenix).
A few variants live on, particularly linux and BSD, and note that Apple OSX is a BSD derivative.
Note that the above refer to things that varied at how the lowest levels were built.
There a a lot of names today that differ only in the software built on top - mostly built on linux and 99% of their lower levels are identical.
We sometimes want a single name for these systems, and what they share in the broader sense.
And these days, we may well refer to POSIX-compliant unices, which makes them share enough to be more easily compatible from a software development perspective.
We tend to use *nix, referring to their eventual pedigree of unix, and the fact that many were named similarly.