Just tell me Why ...
 

Why don't you write Unix? It's always only *NIX ...

A * is to replace the part that changes. You use that syntax when you want to refer to something that works for all *nixes, got it?
Just like when you say win9X, the x is for 95 and 98.

*nix refers to both linux and unix.

the * is called a wildcard. wildcards are interpreted as different things by the shell. a few examples:

* an asterisk matches any number of characters, including none.

? the question mark matches any single character

[] Brackets enclose a set of characters, one of which may match a single character at that position.

...

so *nix could match unix, linux, nix, thisisareallylongandstupidnamenix, etc.
and ?nix could match unix, onix, knix, but not linux

but as michaelk has said, it typically means both unix and linux.

hope this helps.

Oh no it couldn't!!!

*nix would definately NOT match Linux (*nux would)

Carl.

haha, my bad.

the point being all variants of unix...