macOS (pronounced ; previously Mac OS X and later OS X) is the current series of Unix-based graphical operating systems developed and marketed by Apple Inc. designed to run on Apple's Macintosh computers ("Macs"), having been preinstalled on all Macs since 2002. Within the market of desktop, laptop and home computers, and by web usage, it is the second most widely used desktop OS after Microsoft Windows.
Launched in 2001 as Mac OS X, the series is the latest in the family of Macintosh operating systems. Mac OS X succeeded "classic" Mac OS, which was introduced in 1984, and the final release of which was Mac OS 9 in 1999. An initial, early version of the system, Mac OS X Server 1.0, was released in 1999. The first desktop version, Mac OS X 10.0, followed in March 2001. Releases were code named after big cats from the original release up until OS X 10.8 Mountain Lion. Beginning in 2013 with OS X 10.9 Mavericks, releases have been named after landmarks in California. Apple changed the original name "Mac OS X" to "OS X" in 2012 and then to "macOS" in 2016, adopting the nomenclature that it uses for their other operating systems, iOS, watchOS, and tvOS. The latest version of macOS is macOS 10.12 Sierra, which was publicly released in September 2016.
macOS is based on technologies developed at NeXT between 1985 and 1997, when Apple acquired the company. The "X" in Mac OS X and OS X is pronounced "ten", as it is the Roman numeral for the number 10. The X was a prominent part of the operating system's brand identity, and was used to showcase its Unix compatibility; UNIX 03 certification was achieved for the Intel version of Mac OS X 10.5 Leopard and all releases from Mac OS X 10.6 Snow Leopard up to the current version also have UNIX 03 certification. macOS shares its Unix-based core, named Darwin, and many of its frameworks with iOS, tvOS and watchOS. A heavily modified version of Mac OS X 10.4 Tiger was used for the first-generation Apple TV.
Apple also used to have a separate line of releases of Mac OS X designed for servers. Beginning with Mac OS X 10.7 Lion, the server functions were made available as a separate package on the Mac App Store.
Releases of Mac OS X from 1999 to 2005 can run only on the PowerPC-based Macs from the time period. After Apple announced that they were switching to Intel CPUs from 2006 onwards, a separate version of Mac OS X 10.4 Tiger was made and distributed exclusively with early Intel-based Macs; it included an emulator known as Rosetta, which allowed users to run most PowerPC applications on Intel-based Macs. Mac OS X 10.5 Leopard was released as a Universal binary, meaning the installer disc supported both Intel and PowerPC processors. In 2009, Apple released Mac OS X 10.6 Snow Leopard, which ran exclusively on Intel-based Macs. In 2011, Apple released Mac OS X 10.7 Lion, which no longer supported 32-bit Intel processors and also did not include Rosetta. All versions of the system released since then run exclusively on 64-bit Intel CPUs and do not support PowerPC applications.