The original consumer release of Mac OS X arrived in 2001. Apple released Mac OS X 10.0, code-named Cheetah, on March 24, 2001, beginning the modern Unix-based era of the Mac operating system.
Mac OS X replaced the classic Mac OS architecture with technology derived in part from NeXTSTEP and BSD. It introduced the Aqua graphical interface, the Dock, and a Unix foundation intended to improve stability, multitasking, and system-level capabilities. The release followed Mac OS X Public Beta, which gave users and developers an early look at Apple’s new direction.
Cheetah was an important foundation but was widely viewed as an early, incomplete first release. Apple followed it with Mac OS X 10.1, code-named Puma, in September 2001; that update improved performance and added features missing from the initial version. The year 2001 refers to the consumer Mac OS X 10.0 release, not the later renaming of the platform to OS X or macOS.