The Free On-line Dictionary of Computing (30 December 2018):

Mac OS X

    /mak oss ten/ Version 10 of the Macintosh
   Operating System, based on FreeBSD unlike prevoius
   versions.  Apple released the kernel of Mac OS X Server as
   "darwin", under an open source license.  Mac OS X incldues
   a code framework called the "Core Foundation" and an
   "Application Kit" framework for GUI and widgets strongly
   derived from NEXTSTEP.

   (2007-03-15)