Apple Unix

It could run regular Unix programs and X-windows1 programs like other Unix boxes, but it could also run Macintosh programs. Practically all existing Macintosh programs could run in a kind of classic environment. But you could write a special kind of program that lived in the Unix environment, had Unix virtual memory and memory protection, and used the Macintosh toolbox to create its user interface.
Basal Gangster

Way before I started using Macs, I didn’t even know it existed.