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)