Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintoshline of computer systems.

The original operating system was first introduced in 1984 as being integral to the original Macintosh, and referred to as the "System". Referred to by its major revision starting with "System 6 and "System 7", Apple rebranded version 7.6 as "Mac OS" as part of their Macintosh clone program in 1996. The Macintosh, specifically its system software, is credited with having popularized the early graphical user interface concept.

Macintosh operating systems have been released in two major series. Up to major revision 9, from 1984 to 2000, it is historically known as Classic Mac OS. Major revision 10, from 2001 to present, is branded OS X (originally referred to as Mac OS X).^[2] Major revisions to the Macintosh OS are now issued as point revisions, such that, for example, 10.2 is substantially different from 10.5. Both series share a general interface design, and there has been some overlap with shared application frameworks and virtual machine technology for compatibility; but the two series also have deeply different architectures.