MacOS (the Macintosh operating system, known initially simply as "System Software" and renamed first "Macintosh System Software" in 1987 with System 5 and finally with its definitve name in 1986, with MacOS 7.6), also known as "classic MacOS" to distinguish it from the later macOS, was a personal computer operating system created by Apple Computer for use in its Macintosh computer systems, which were available for sale from January 24th, 1984. The classic MacOSes (versions 1 through 9) were cooperative multi-tasking operating systems, and were often known amongst computer enthusiasts for their propensity for regular crashes, which required full reboots of the computer systems in order to recover from, although they were also known for their technical superiority when compared to their main competitor, Microsoft Corporation's MS-DOS and other similar DOS OSes running on IBM PC compatibles. The superiority was less apparent, if existant, when compared to Microsoft's later Windows operating systems.
MacOS exclusively ran on computers mounting Motorola 68xxx CPUs until System 7.2 (1994), which began support for IBM's PowerPC CPUs. The system was discontinued in favor of Mac OS X (later renamed "OS X" and "macOS") in 2009, when Apple, Inc. transitioned from PowerPC processors to more standard Intel x86 ones.
Arguably, from a purist user's standpoint, the toughest part of Apple's transition from MacOS "Classic" to Mac OS X on Intel CPU's is the loss of the ability to run "Classic" apps on the Intel-based systems. As a workaround, however, emulation software such as Basilisk II or SheepShaver have emerged to allow greater or lesser degrees of support for such legacy applications.