OS X ( /oʊ ˌɛs ˈtɛn/), formerly Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. OS X (officially) runs exclusively on Macintosh computers and has been pre-loaded on all Macs since 2002.

OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. It was the successor to Mac OS 9, released in 1999, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984.

OS X is a UNIX-like operating system that originally ran on PowerPC-based Macs. In 2006, the first Intel Macs had a specialized version of Mac OS X v10.4 "Tiger". in 2007, Mac OS X 10.5 "Leopard", was the first to have UNIX 03 certification and run on both PowerPC and Intel Macs with the use of Universal Binaries. Mac OS X 10.6 "Snow Leopard" was the first version of OS X to drop support for PowerPC Macs and run solely on Intel's x86-based processors. Mac OS X 10.7 "Lion" was the first version of OS X to drop support for 32-bit processors and run exclusively on 64-bit CPUs.

Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface. The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.

From the beginning, Apple deliberately sought to minimize by design the user's conceptual awareness of the operating system as such. Tasks that on other products required a more explicit working knowledge of an operating system would on a Macintosh be accomplished by intuitive mouse gestures and manipulation of graphical control panels. The intention was that the product would thus be more user-friendly and so more easily mastered. This would differentiate it from devices using other operating environments, such as MS-DOS machines, which were more technically challenging to operate.