Cheetah

Mac OS X version 10.0, code named Cheetah, is the first major release of Mac OS X, Apple’s desktop and server operating system. Mac OS X v10.0 was a radical departure from the previous classic Macintosh operating system (Mac OS) and was Apple’s long awaited answer for a next generation Macintosh operating system. It introduced a brand new code base completely separate from Mac OS 9's, as well as all previous Apple operating systems. Mac OS X introduced the new Darwin Unix-like core and a completely new system of memory management. Cheetah proved to be a rocky start to the Mac OS X line, plagued with missing features and performance issues, although it was praised for being a good start to an operating system still in its infancy, in terms of completeness and overall operating system stability. Unlike later releases of Mac OS X, the cat-themed code name was not used in marketing the new operating system.