macOS version history (Classic Mac)
The "Classic" Mac OS is a graphical user interface-based operating system developed by Apple Inc. for its Macintosh line of personal computers from 1984 until 2001, the original member of the family of Macintosh operating systems. The Macintosh platform, which was introduced in the classic Mac OS, is credited with having popularized the early GUI concept. Mac OS was preinstalled on every Macintosh computer that was made during the era it was developed; it was also sold separately in retail stores.
Apple released the original Macintosh on January 24, 1984. Its early system software was partially based on the Lisa OS, previously released by Apple for the Lisa computer in 1983; as part of an agreement allowing Xerox to buy shares in Apple at a favorable price, it also used concepts from the Xerox PARC Alto computer, which former Apple CEO Steve Jobs and other Macintosh team members had previewed. The operating system integral to the Macintosh was originally named System Software, or simply "System", and referred to by its major revision starting with System 6 and System 7. Apple rebranded the system as Mac OS in 1996, starting officially with version 7.6, due in part to its Macintosh clone program. That program ended after the release of Mac OS 8 in 1997. The last major release of the system was Mac OS 9 in 1999.
Mac OS is characterized by its monolithic system. From its original release through System 4, it ran only one application at a time. Even so, it was noted for its ease of use. Mac OS gained cooperative multitasking with System 5, which ran on the Macintosh SE and Macintosh II. It was criticized for its very limited memory management, lack of protected memory, no access controls, and susceptibility to conflicts among extensions that provide additional functionality such as networking or support for a particular device.
After a four-year development effort spearheaded by Steve Jobs' return to Apple in 1997, Apple replaced Mac OS with a new operating system in 2001 named Mac OS X; the "X" represented the tenth major revision of the Mac system software as well as its history as part of NeXT and its relation to Unix. Mac OS X was renamed "OS X" in 2012 and "macOS" in 2016. The general interface design of the current macOS shares its legacy with the classic Mac OS, and there was some overlap of application frameworks for compatibility, but the two systems have different origins and use deeply different architectures.
The final updates to Mac OS 9 released in 2001 provided interoperability with Mac OS X. The name "Classic" that now signifies the historical Mac OS as a whole is a reference to the Classic Environment, a compatibility layer that helped ease the transition to Mac OS X.