OS X (pronounced /oʊˌɛsˈtɛn/, oh-ess-ten; originally Mac OS X) is a series of Unix-based graphical interface operating systems (OS) developed and marketed by Apple Inc. It is designed to run on Macintosh computers, having been pre-installed on all Macs since 2002. OS X is the fourth most popular general purpose OS; within the market of desktop, laptop and home computers, and by web usage, OS X is the second most widely used desktop OS after Windows.
OS X 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. The first version released was Mac OS X Server 1.0 in 1999; a desktop version, Mac OS X v10.0 “Cheetah” followed on March 24, 2001. All consumer releases of OS X up to 2013 were named after big cats; for example, OS X v10.8 was referred to as “Mountain Lion”. However, with the announcement of OS X Mavericks in June 2013, this was dropped in favor of California landmarks.
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. The ‘X’ is also used to emphasize the relatedness between OS X and UNIX. UNIX 03 certification has been achieved for versions 10.5 for Intel CPUs, and versions 10.6 through 10.11. iOS, the mobile OS for the iPhone, iPod Touch, iPad, and the 2nd and 3rd generation Apple TV, shares the Unix-based core and many frameworks with OS X. An unnamed variant of v10.4 powers the first generation Apple TV. Apple also formerly issued a separate line of editions for server use.
The first releases of Mac OS X from 1999 to 2006 can run only on the PowerPC based Macs of the period. After Apple announced it would shift to using Intel x86 CPUs from 2006 onwards, Tiger and Leopard were released in versions for Intel and PowerPC processors. Snow Leopard is the first version released only for Intel Macs. Since the release of Mac OS X 10.7 “Lion”, OS X has dropped support for 32-bit Intel processors as well. It now runs exclusively on 64-bit Intel CPUs.
The latest version of OS X is 10.11 “El Capitan”, which was released to the public on September 30, 2015.
(Source – Wikipedia)