OS X Mountain Lion is the ninth major release of OS X, 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 Apple's Mac App Store, as part of a switch to releasing OS X versions online and every year. 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 to users of Apple's more popular mobile operating system, iOS.
The operating system gained the new malware-blocking system Gatekeeper and integration with Apple's online Game Center and iCloud services, while the Safari web browser was updated to version 6. As on iOS, Notes and Reminders became full applications, separate from Mail and Calendar, while the OS X iChat application was replaced with a version of iOS's Messages. Mountain Lion also added a version of iOS's Notification Center, which groups updates from different applications in one place.
Integrated links allowing the user to rapidly transfer content to Twitter were added to the operating system from launch. Facebook integration was also planned but unfinished at launch date, and released as a downloadable update later. OS X Mountain Lion received positive reviews, with critics praising Notification Center, Messages, and speed improvements over Mac OS X Lion, while criticizing iCloud for unreliability and Game Center for lack of games. Mountain Lion sold three million units in the first four days, making it Apple's most successful Mac OS release to date