Mac OS X (Mac OS Ten) - the operating system of Apple Computer, indoor and significantly revised version of the BSD-UNIX Berkeley computers available for Macintosh (Mac) PowerPC-based and Intel.Mac OS X is significantly different from previous versions of Mac OS.Mac OS X is significantly different from previous versions of Mac OS. The basis of the system - Darwin. The basis of the system - Darwin. Darwin - a free software with open source. Darwin - a free software with open source. Its kernel is XNU (an acronym for Xnu Not Unix - Xnu not mean Unix"), which uses the Mach kernel and standard services BSD (descended from FreeBSD 5).