The operating system distributed with the Macintosh personal computer since 2001. Derived from the NEXTSTEP operating system, which Apple acquired when it bought NeXT Software, Inc., in 1996, it combined a development of the traditional Macintosh graphical user interface built over a UNIX base. The latter provided such important technical improvements as pre-emptive multitasking and memory protection. Since the first version 10.0 ‘Cheetah’ (continuing the numbering of the previous Macintosh OS), there have been 10 versions, the latest being 10.10 ‘Yosemite’.