*nix is a convention that refers to all conventions of Unix, including GNU/Linux and the BSD derivatives.
From Wikipedia: (http://en.wikipedia.org/wiki/%2Anix)
Some add a wildcard character to the name to make a euphemistic abbreviation like Un*x or *nix, since Unix-like systems often have Unix-like names such as AIX, IRIX, Linux, Minix, Ultrix, and Xenix. These patterns do not literally match many system names, but are still generally recognized to refer to any UNIX descendant system, even those with completely dissimilar names such as Solaris or FreeBSD.
From nixdoc.net:
The *nix documentation project is meant to provide online searchable documentation for Linux, FreeBSD, OpenBSD, NetBSD, HP-UX, Tru64 and other Unix like operating systems.
A search on Slashdot for "*nix" returns many, many results for this term.
If you prefer, this snip of Perl, should take care of this:
$assignment =~ s/*nix/A variant of the UNIX operating system/g;
Mark Walling