Ī prominent part of macOS's original brand identity was the use of Roman numeral X, pronounced 'ten' as in Mac OS X and also the iPhone X, as well as code naming each release after species of big cats, or places within California. Apple's mobile operating system, iOS, has been considered a variant of macOS. All releases from Mac OS X 10.5 Leopard and after are UNIX 03 certified, with an exception for OS X 10.7 Lion. The first desktop version, Mac OS X 10.0, was released in March 2001, with its first update, 10.1, arriving later that year. During this time, Apple cofounder Steve Jobs had left Apple and started another company, NeXT, developing the NeXTSTEP platform that would later be acquired by Apple to form the basis of macOS. MacOS succeeded the classic Mac OS, a Mac operating system with nine releases from 1984 to 1999. Within the market of desktop and laptop computers it is the second most widely used desktop OS, after Microsoft Windows and ahead of ChromeOS. It is the primary operating system for Apple's Mac computers.

MacOS ( / ˌ m æ k oʊ ˈ ɛ s/ previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc.

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