System 7 (whose codename being "Big Bang", reflects on the considerable changes that came with the OS) is the term used to refer to the Mac OS that superseded the earlier versions known simply as "The System", or "System 6", and before the use of the term "Mac OS" came into official use. The name reflects the version number of the OS—7.0, but the term is also used to apply to all the 7.x versions. System 7.0 was released for the Apple Macintosh on May 13, 1991.