Mac OS X Lion (version 10.7; marketed as OS X Lion) is the eighth and current major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers.

A preview of Lion was publicly unveiled at Apple's "Back to the Mac" event on October 20, 2010. It brings many developments made in Apple's iOS, such as an easily navigable display of installed applications, to the Mac, and includes support for the Mac App Store, as introduced in Mac OS X Snow Leopard version 10.6.6. On February 24, 2011, the first developer's preview of Lion (11A390) was released to subscribers of Apple's developers program. Other developer previews were subsequently released, with Lion Preview 4 (11A480b) being released at WWDC 2011.

Lion achieved golden master status on July 1, 2011, followed by its final release via the Mac App Store on July 20, 2011. Apple reported over 1 million Lion sales on the first day of its release. As of October 2011, Mac OS X Lion has sold over 6 million copies worldwide.

OS X ( /oʊ ˌɛs ˈtɛn/), formerly Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. OS X (officially) runs exclusively on Macintosh computers and has been pre-loaded on all Macs since 2002.

OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. 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.

OS X is a UNIX-like operating system that originally ran on PowerPC-based Macs. In 2006, the first Intel Macs had a specialized version of Mac OS X v10.4 "Tiger". in 2007, Mac OS X 10.5 "Leopard", was the first to have UNIX 03 certification and run on both PowerPC and Intel Macs with the use of Universal Binaries. Mac OS X 10.6 "Snow Leopard" was the first version of OS X to drop support for PowerPC Macs and run solely on Intel's x86-based processors. Mac OS X 10.7 "Lion" was the first version of OS X to drop support for 32-bit processors and run exclusively on 64-bit CPUs.