The asterisk is there to avoid writing the word out in full, like "G*d" or "f*ck". There was a time when Unix[1][2] was a trademarked name and if you used it you had to attribute it. If you wanted to refer to the general family of Unix/POSIX/SysV/BSD/etc systems, you might be tempted to write "Unix"[1][2], but to avoid the presumably-Sauron[3]-like eye of the trademark holders, you'd bowdlerize it a little.

[1] Unix is a registered trademark of AT&T Bell Labs.

[2] Unix is a registered trademark of The Open Group.

[3] Sauron is a registered trademark of Tolkien Enterprises