macOS (formerly Mac OS X) is an operating system for Apple Macintosh computers, first released to the public on March 24, 2001, developed by Apple. It is the successor to Mac OS 9, hence the X signifying both its Unix roots and the major release version number 10. As mentioned by Apple, Wikipedia, and others, it is said as Mac OS 10. It shares none of the "Classic" Mac OS design, and is completely rewritten and uses Next frameworks, a hybrid XNU/Mach kernel, and a BSD subsystem dubbed "Darwin". While underlying components of OS X are free/open source software, the top layers, such as the Aqua UI, are proprietary; Darwin packages can be downloaded and compiled from the Apple Open Source website to make a bootable OS.
Mac OS X has been built for three different architectures and four platforms during its release cycle to date. The first six releases (10.0.0-10.5.8) were designed for the PowerPC architecture, adding 64-bit PowerPC support as an additional platform for the G5 in 10.3 Panther. Intel (x86) support started with 10.4.4 Tiger, and was built as a universal release for both PowerPC/x86 with 10.5 Leopard, which finally dropped all G3 support. Since 10.6, PowerPC support is non-existent/dropped, and Mac OS X is currently designed for Mac computers with Intel 32-bit (x86) and Intel 64-bit (x86_64) architectures. AMD is not currently officially supported. Starting with 10.7 "Lion", Mac OS X is now referred to simply as "OS X".
The "iPhone OS" or iOS, which powers the iPhone, iPod touch, and iPad is a direct descendant of OS X, and shares its design and many internal frameworks. The previous version of OS X is "Yosemite" (10.10), released on October 16, 2014. The latest version is 12.2.1. 
Screenshots of this build were shown in the BYTE magazine in 1984. It has a black default background and has the Arrange menu.
- Main article: System 0.85
- Main article: System 0.97
An update to 0.97, had slight changes but is otherwise the same. The most common version of pre-System 6 OS'es.
This version came up on 2 disks packed with developing / debugging software. This used to be a rare version until it was later leaked on BetaArchive.
Technically the same as System 1.1, yet has several bug fixes.
System 6 was released in April 1988. Cooperative multitasking made its Macintosh debut in March 1985 with a program called Switcher, which allowed the user to launch multiple applications and switch between them. However, many programs and features did not function correctly with Switcher, and it did not come with the operating system, so it had to be acquired from Apple separately. System 6 featured a much more seamless approach called MultiFinder. MultiFinder originally debuted with System 5 (System file 4.2 / Finder 6.0). Multitasking under System 6 was optional — startup could be set to Finder or MultiFinder. If MultiFinder was selected, the Finder and its functions continued to run when an application was launched. The MultiFinder environment allowed users to see past the windows of running applications to view Finder icons such as the Trash, or the windows of other applications running in the background.
Mac OS 7
Mac OS 7 (codenamed Big Bang) was initially released in May 1991.
Versions 7.0 through 7.1.1 worked only on the 68k platform. Later, version 7.1.2 added support for PowerPC processors.
Copland was the codename for an unreleased Mac OS version that was intended to be the modernized successor to Mac OS 7.
The Copland project started in March 1994 and was eventually cancelled in August 1996.
Mac OS 8
Mac OS 8 (codenamed Tempo) was released on the 26th of July 1997.
Initially, the early beta releases of the product which were circulated to developers and Apple internal audiences, were branded as Mac OS 7.7 (superseding the current release, Mac OS 7.6). Afterwards, the software was later renamed to Mac OS 8 before the final release. The first two releases of Mac OS 8 can run on Motorola 68k processors, however version 8.5 dropped support for the 68k platform, only supporting PowerPC-based models.
Mac OS 9
Mac OS 9 (codenamed Sonata) was released on the 23th of October 1999. Its development had been discontinued in May 2002.
Mac OS X: 2001-2015
|Release||Version||Internal name||Architecture||Additional information|
|Mac OS X Server 1.x||1.0-1.2.3||Rhapsody 5.3||G3 Beige - early G4 (ppc)||Early developer releases of Mac OS X based on the Rhapsody OS.|
|Mac OS X Public Beta||10.0.1H39||Kodiak||G3 Beige - G4 (ppc32)||Official beta for participating users; famously had no Apple menu.|
|Mac OS X Cheetah||10.0||Cheetah||G3 Beige - G4 (ppc32)||The gold release of Mac OS X. While revolutionary, Cheetah was slow and lacked labels, burn support, and other features.|
|Mac OS X Puma||10.1||Puma||G3 Beige - G4 (ppc32)||Incremental update to 10.0, which fixed bugs, optimized the system, and added Burn support. Offered free to affected 10.0 users at the time.|
|Mac OS X Jaguar||10.2||Jaguar||G3 Beige - G4 (ppc32)||First major upgrade for Mac OS X, with a marketed 150 new features. It is also the first to sport a feline theme and its codename on the box.|
|Mac OS X Panther||10.3||Panther||G3/G4 (ppc32), G5 (ppc64)||Second major upgrade for Mac OS X. Introduced Expose, FileVault, rapid search APIs, G5 support, and a new Finder.|
|Mac OS X Tiger||10.4||Tiger||G3/G4 (ppc32), G5 (ppc64), x86/x64||Introduced Spotlight, Dashboard, H.264 support, and was the first to run on x86 (10.4.7+). It is the longest running release ever with 11 updates.|
|Mac OS X Leopard||10.5||Leopard||G4 (ppc32), G5 (ppc64), x86/x64||Introduced Cocoa Finder with QuickLook, Spaces, Time Machine, and visual overhaul. Last version to support G4/G5, and only unified x86/x64/ppc(64) release on one disc.|
|Mac OS X Snow Leopard||10.6||Snow Leopard||Intel x86/x64||Optimized $29 successor to 10.5. It is the last version for 32-bit x86 (Core Solo/Duo).|
|Mac OS X Lion||10.7||Lion||Intel x64||First release to require x64, and the first digital (non-optical) release. Introduced Autosave, fullscreen app support, Mission Control, the Mac App Store, Launchpad, and many other features for $19.99. It is the last release to not require efi64 (late 2008 and later).|
|OS X Mountain Lion||10.8||Mountain Lion||Intel x64||$19.99 upgrade to Lion, with Gatekeeper, better memory protection, improved scrolling and Autosave control, tweaked applications, and new iOS inspired applications (Notes, Reminders). Dropped 'Mac' in the OS X title, and requires an x64 EFI, which obsoleted several 64-bit Macs.|
|OS X Mavericks||10.9||Mavericks||Intel x64||First release to drop the feline theme, named after California landmarks. First release of OS X since 10.1 to be free to Mac users. Introduced major core system improvements since Snow Leopard, including timed coalescing, memory compression, and energy tweaks.|
|OS X Yosemite||10.10||Yosemite||Intel x64||Features a redesign of the UI to match iOS 7, Swift, an all-new Spotlight, Handoff support, Continuity, widgets, and more. Adds Extensions (ode to classic), and a dark mode for the Dock and Menubar.|
|OS X El Capitan||10.11||El Capitan||Intel x64||Introduces filters for Spotlight, SIP, and overall improvements to the system like 10.6 and 10.9.|
|Release||Version||Internal name||Architecture||Additional information|
|macOS Sierra||10.12||Sierra||Intel x64||Visually changes the name for the first time since 2001. It has added Siri to the Mac, Optimized Storage, watchOS paired unlocking, improved Swift, universal Clipboard and Tabs, and APFS support. It is the first release since 10.8 to shift requirements.|
|macOS High Sierra||10.13||High Sierra|
|macOS Mojave||10.14||Mojave||Ends support for OpenGL and OpenCL in favour of Apple's proprietary Metal graphics API. Some devices can forced to install this version by change Metal-GPUs.|
|macOS Catalina||10.15||Catalina||Replaces iTunes with three new apps: Apple Music, Apple Podcasts, and Apple TV. Ends all support for 32-bit applications.|
|macOS Big Sur||11.0||Big Sur||Intel x64, ARM64|
There are few 68k Macintosh emulators available today, which have been ported to various platforms and operating systems.
One of them is a PowerPC emulator called "SheepShaver", which runs Mac OS 7.1.2 through 9.0.4, and has been ported to various operating systems. The reason Mac OS 9.1 and up won't run in SheepShaver is that SheepShaver lacks a Memory Management Unit emulator, something that the last few Mac OS releases need.