The definitive response on what makes a UNIX® system a UNIX system can be found straight from the proverbial horse's mouth at http://www.unix-systems.org . To be a UNIX system, an operating system must have these qualities:
More information on The Open Group's Open Brand program is available at http://www.opengroup.org/registration/ . UNIX® is capitalized for the same reason STAR WARS® and SPAM® are capitalized in press releases; they're trademarks.
See also The UNIX Trademark.