Mac OS X 10.0 (codenamed Cheetah) is the first release of macOS, shipped on March 24, 2001, for $129 — the moment Apple replaced the aging Classic Mac OS with a modern operating system built from NeXT's Unix foundations, the fruit of the 1997 acquisition that returned Steve Jobs to Apple. Atop the Darwin core and XNU kernel sat the Aqua interface — glossy, translucent, animated — with the Dock, protected memory, and preemptive multitasking ending the era of one crashed app taking down the Mac.

It was, by broad agreement, not ready: slow even on new hardware, missing DVD playback and CD burning, with scarce native apps (users lived in the Classic environment compatibility layer). Apple sold it to pioneers and kept Mac OS 9 as the default boot.

Aqua, the Dock, the column-view Finder, Mail, TextEdit, protected memory, symmetric multiprocessing, Quartz 2D compositing, and the full Unix terminal underneath — the architecture that still runs every Mac, iPhone, and Apple device today.

Cheetah's importance is entirely foundational: reviewers panned the speed and gaps while conceding the base was the future. Its free 10.1 successor fixed the worst within six months, and the Darwin/Aqua architecture it introduced now spans six Apple platforms — the longest-lived OS foundation in consumer computing.