OS X Mountain Lion is the ninth major release of macOS, Apple Inc.'s desktop and server operating system for Macintosh computers. OS X Mountain Lion was released on July 25, 2012, for purchase and download through the Mac App Store, as part of a switch to releasing OS X versions online and every year, rather than every two years. Named to signify its status as a refinement of the previous OS X version, Lion, Apple's stated aims in developing Mountain Lion were to allow users to more easily manage and synchronise content between multiple Apple devices and to make the operating system more familiar.

macOS, originally Mac OS X, previously shortened as OS X, is an operating system developed and marketed by Apple since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and laptop computers, it is the second most widely used desktop OS, after Microsoft Windows and ahead of all Linux distributions, including ChromeOS.