From Mac Guides
A/UX, Apple UNIX, was an implementation of System V UNIX by Apple. It ran on early Macintoshes like the Macintosh II, Quadra and Centris. Despite being updated up to 1995, A/UX was only compatible with Motorola 68k processors with a floating point unit and a paged memory managment unit.
There was an application layer that enabled the execution of pure Macintosh applications on A/UX. However, programmers could create programs that were hybrid, taking advantage of the Macintosh toolbox while also taking advantage of the inherited UNIX features.
There may be some confusion over Apple's Network Server 500 and 700 Models (introduced in 1996) that ran AIX (an IBM flavor of UNIX). AIX is the only operating system other than the Mac OS that Apple has ever shipped preinstalled onto a Macintosh or Server.