Redirected from Mac OS X
For a long time the official name of the OS was "Mac OS X" (pronounced "Mac OS Ten"). There were few who actually referred to it this way. Common usage was often "OS X" (either pronounced as-is or as "OS Ten").
When Sierra came out, Apple switched the terminology again, this time to "macOS", and another step closer to the days before Mac OS X, when the predecessor operating system had been called "Mac OS".