A/UX is Apple Computer's implementation of the Unix operating system for some of their Macintosh computers, the latest versions running on the Mac II, Quadra and Centris series of machines.
The system is based on System V.2.2 with features of V.3 and V.4 and BSD 4.2 and 4.3. It is POSIX and System V Interface Definition (SVID) compliant.
A/UX has the Macintosh operating system Mac OS version 7.0.1 running on top of it, so the user is virtually using a normal Macintosh, with the familiar Finder graphical user interface and applications. An X Window System server application or a terminal program may be used to interface with the underlying Unix-system. Alternatively the user may choose to run a full X11R4 session, without the Finder.
Unfortunately for A/UX fans, the Unix niche was a costly business in the early 1990s. A/UX was never ported to PowerPC Macintoshes, and Apple all but abandoned it by 1994. As Steve Jobs returned to Apple later on, the future of Mac OS would end up truer to NeXTSTEP than to the old Mac or traditional Unix.