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An Unix-based operating system originally forked from NextSTEP and developed by Apple Inc. MacOS X runs a Darwin kernel together with Apple Inc. utilities.

From Wikipedia:

OS X previously Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. It is designed to run exclusively on Mac computers, having been pre-loaded on all Macs since 2002. It was the successor to Mac OS 9, released in 1999, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984. The first version released was Mac OS X Server 1.0 in 1999, and a desktop version, Mac OS X v10.0 "Cheetah" followed on March 24, 2001. Releases of OS X are named after big cats: for example, OS X v10.8 is referred to as "Mountain Lion".