macOS, originally named Mac OS X, is an operating system first released by Apple on March 16, 1999, for servers. It was later released for desktop computers on March 24, 2001, and is a completely new Apple Macintosh operating system based on BSD.
Apple code-named each release of the macOS using names of big cats or places in California, except macOS Server 1.0. For a listing of the macOS releases, including codenames, see our Apple OS history.
Some may abbreviate macOS X as OSX.