From 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)