Lion is the marketing name for OS X v10.7, the seventh major upgrade to OS X. As of this release, Mac was dropped from the marketing name of "Mac OS X".
Mac OS X Lion (known as "OS X Lion" in all marketing materials) is the eighth major version of OS X, and the seventh major upgrade to the OS since Mac OS X 10.0.
It was released on Wednesday July 20th 2011 via the Mac App Store for $29.99/£20.99. As of July 25th, 2012, Lion is no longer available on the App Store, and was replaced by its successor, mountain-lion.
Major upgrades include various iOS inspired interface changes such as Launchpad, an app launcher; the transition from Expose to Mission Control; new multi-touch gestures; integrated autosave and Versions, which allows users to manage versions of saved files. The first developer preview was released on February 24, 2011 with a second version being released at the 2011 WWDC.
System Requirements for OS X Lion: