macOS 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 codenamed each release of the macOS X operating system using names of big cats or places in California, with the exception of macOS X Server 1.0. A listing of the macOS X releases, including codenames, can be found on our Apple OS history page.
Tip: OS X is pronounced as "OS 10" and not "OS X."