Mac OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is a Unix-based graphical operating system, built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. From its sixth release, Mac OS X v10.5 "Leopard" and onward, every release of Mac OS X gained UNIX 03 certification while running on Intel processors.
The first version released was Mac OS X Server 1.0 in 1999, and a desktop-oriented version, Mac OS X v10.0 "Cheetah" followed on March 24, 2001. Releases of Mac OS X are named after big cats: for example, Mac OS X v10.7 is usually referred to by Apple and users as "Lion". The server edition, Mac OS X Server, is architecturally identical to its desktop counterpart, and includes tools to facilitate management of workgroups of Mac OS X machines, and to provide access to network services. These tools include a mail transfer agent, an LDAP server, a domain name server, and others. It is pre-loaded on Apple's Xserve server hardware, but can be run on almost all of Apple's current selling computer models.
Apple also produces specialized versions of Mac OS X for use on its consumer devices. iOS, which is based on Mac OS X, runs on the iPhone, iPod Touch,iPad, and the 2nd generation Apple TV.An unnamed variant of Mac OS X powered the 1st generation Apple TV.
The APIs that Mac OS X inherited from OpenStep are not backward compatible with earlier versions of Mac OS. These APIs were created as the result of a 1993 collaboration between NeXT Computer and Sun Microsystems and are now referred to by Apple as Cocoa. This heritage is highly visible for Cocoa developers, since the "NS" prefix is ubiquitous in the framework, standing variously for Nextstep or NeXT/Sun. The official OpenStep API, published in September 1994, was the first to split the API between Foundation and Application Kit and the first to use the "NS" prefix. Apple's Rhapsody project would have required all new development to use these APIs, causing much outcry among existing Mac developers. All Mac software that did not receive a complete rewrite to the new framework would run in the equivalent of the Classic environment. To permit a smooth transition from Mac OS 9 to Mac OS X, the Carbon Application Programming Interface (API) was created. Applications written with Carbon can be executed natively on both systems. Carbon was not included in the first product sold as Mac OS X, Mac OS X Server (now known as Mac OS X Server 1.x).
Mac OS X also used to support the Java Platform as a "preferred software package" – in practice this means that applications written in Java fit as neatly into the operating system as possible while still being cross-platform compatible, and that graphical user interfaces written in Swing look almost exactly like native Cocoa interfaces. Traditionally, Cocoa programs have been mostly written in Objective-C, with Java as an alternative. However, on July 11, 2005, Apple announced that "features added to Cocoa in Mac OS X versions later than 10.4 will not be added to the Cocoa-Java programming interface."
Since Mac OS X is POSIX compliant, many software packages written for the *BSDs, Linux, or other Unix-like systems can be recompiled to run on it. Projects such as Homebrew, Fink, MacPorts and pkgsrc provide pre-compiled or pre-formatted packages. Since version 10.3, Mac OS X has included X11.app, Apple's version of the X Window System graphical interface for Unix applications, as an optional component during installation. Up to and including Mac OS X v10.4 (Tiger), Apple's implementation was based on the X11 Licensed XFree86 4.3 and X11R6.6. All bundled versions of X11 feature a window manager which is similar to the Mac OS X look-and-feel and has fairly good integration with Mac OS X, also using the native Quartz rendering system. Earlier versions of Mac OS X (in which X11 has not been bundled) can also run X11 applications using XDarwin. With the introduction of version 10.5 Apple switched to the X.org variant of X11. Version Mac OS X 10.7 "Lion" use X.org Server version 1.10.
For the early releases of Mac OS X, the standard hardware platform supported was the full line of Macintosh computers (laptop, desktop, or server) based on PowerPC G3, G4, and G5 processors. Later versions discontinued support for some older hardware; for example, Panther does not support "beige" G3s,and Tiger does not support systems that pre-date Apple's introduction of integrated FireWire ports (the ports themselves are not a functional requirement). Mac OS X v10.5 "Leopard", introduced October 2007, has dropped support for all PowerPC G3 processors and for PowerPC G4 processors with clock rates below 867 MHz. Mac OS X v10.6 "Snow Leopard" supports Macs with Intel processors, not PowerPC. Mac OS X 10.7 "Lion" requires a Mac with an Intel Core 2 Duo or newer processor.
Tools such as XPostFacto and patches applied to the installation disc have been developed by third parties to enable installation of newer versions of Mac OS X on systems not officially supported by Apple. This includes a number of pre-G3 Power Macintosh systems that can be made to run up to and including Mac OS X 10.2 Jaguar, all G3-based Macs which can run up to and including Tiger, and sub-867 MHz G4 Macs can run Leopard by removing the restriction from the installation DVD or entering a command in the Mac's Open Firmware interface to tell the Leopard Installer that it has a clock rate of 867 MHz or greater. Except for features requiring specific hardware (e.g. graphics acceleration, DVD writing), the operating system offers the same functionality on all supported hardware.PowerPC versions of Mac OS X prior to Leopard retain compatibility with older Mac OS applications by providing an emulation environment called Classic, which allows users to run Mac OS 9 as a process within Mac OS X, so that most older applications run as they would under the older operating system. Classic is not supported on Intel-based Macs or in Mac OS X v10.5 "Leopard", but users still requiring Classic applications on Intel Macs can use the SheepShaver emulator to run Mac OS 9 on top of L