Wikipedia
MachTen is a Unix-like operating system from Tenon Intersystems. It is based on 4.4BSD and the Mach kernel, and features the X Window System and GNU programming tools. It runs only as a classic Mac OS application program (in a virtual machine) on Apple Macintosh computers.
MachTen development started in 1989, culminating in the first release in 1991. The Professional MachTen branch, intended for 68000-based Macintoshes, ended with release 2.3. The Power MachTen branch, which is Power Macintosh compatible, lacks some of the features of Professional MachTen (including true virtual and protected memory models), but takes full advantage of the PowerPC processor and is compatible with Mac OS 9 through its final version, 4.1.4. MachTen is no longer developed, and is functionally superseded by Mac OS X.