Quote:
"UNIX" is a trademark of The Open Group and, like all trademarks, should be used as an adjective followed by a generic term such as "system." The term refers more to a class of operating systems than to a specific implementation of an operating system; those operating systems which meet The Open Group's Single UNIX Specification should be able to bear the "UNIX" and UNIX98 trademarks today. UNIX systems include AIX, HP-UX, IRIX, Solaris, Tru64, A/UX and a part of z/OS. In practice, the term, especially when written as "UN*X", "*NIX", or "*N?X" is applied to a number of other multiuser POSIX-based systems such as GNU/Linux, Mac OS X, FreeBSD, NetBSD, OpenBSD that do not seek UNIX branding because the royalties would be too expensive for a product marketed to consumers or freely available over the Internet.
The term "Unix" is also used, and in fact was the original capitalisation, but the name UNIX stuck because, in the words of Dennis Ritchie "[when presenting the original UNIX paper to the third Operating Systems Symposium of the American Association for Computing Machinery], we had just acquired a new typesetter and were intoxicated by being able to produce small caps" (quoted from the Jargon File, version 4.3.3, 20 September 2002). Additionally, it should be noted that many of the operating system's predecessors and contemporaries used all-uppercase lettering, so many people wrote the name in upper case due to force of habit.