I've noticed that throughout the Internet, within forums and blog posts, Unix always has a * in the word, whether it is *nix or Un*x, as I noticed at the welcoming banner at the Unix StackExchange site.
Why is this like this?
Most of these answers are far too late to the game, as the
* usage was used on Usenet and elsewhere to refer to the multiplicity of Unixoid systems. This was significantly before "the suits" even knew what was happening in cyberspace and didn't understand it if they did.
I found a reference in the comp.risks archive dated May 1987 where the title
Concerning UN*X (in)security
was already so pedestrian as to warrant no explanation. By this time Xenix had been long on the market as were various "*ix" based variants which were decidedly "unix" but not "Unix".
To officially be called a "Unix", an OS has to adhere to the Single UNIX Specification put out by the Open Group. OSes that are similar to Unix without actually complying with the SUS are typically called "Unix-like", which is sometimes abbreviated as Un*x or *nix (since the OSes often have -ix or -nix suffixes: Linux, Minix, IRIX, etc.). Not specifically using "Unix" when referring to them also helps avoid trademark problems
Unix ® is registred as a trademark and was owned by several companies, such as
Nobody likes to read documents like the 12-page trademark usage guide by the open group (pdf).
So the easiest way to refer to Unix was Un*x or *nix. The ® is not really necessary, but its simply a habit.
The trademarked term "UNIX" actually refers to an actual thing, as described by others in this post.
The general community lacks a good term for the class of product which we call Unix, Linux, BSD, BSD, Solaris, etc. These products are related and have a lot in common. A CentOS administrator can perform many tasks on a Solaris or FreeBSD system, for example. Also, many people have never heard of Solaris or FreeBSD, but they've heard of "Unix". So, some people use the term "Unx" to refer to this class of software.
I also think that "Unx" is a humorous word. How do you verbally pronounce "Unx", for example?
I noticed at the welcoming banner at the Unix StackExchange site
I created that wording, so I'll tell you why I did it. I used "Un*x" because I thought it was clever and funny, and I wanted to answer questions like "Does this include things like $MY_FAVORITE_UNIXLIKE_PRODUCT?".
As @echox noted, Unix is a registered trademark. Traditionally, Unix types loathed trademarks and the like. This extended to Linux, which was not originally trademarked. At least not until one William Della Croce registered Linux as a trademark for himself in the mid-nineties - when Linux started to become popularly known and used in trade.
Della Croce then sent cease-and-desist letters to anyone who used the Linux mark. Only then did the Linux community rally and take legal action to claim the trademark. Linus is now the holder of the Linux trademark, but it was not his original wish.
That is how much trademarks were reviled by the Unix crowd.
See https://web.archive.org/web/20081015181133/http://www.informationweek.com/blog/main/archives/2007/05/who_owns_linux.html for details on the Linux trademark.