macOSEdit

Considered by many to be the best operating system in the world (and by me to be the best desktop operating system in the world), macOS is Apple’s current flagship operating system.

See: http://www.apple.com/macos/

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").

With the release of Mountain Lion Apple started referring to it as "OS X" in marketing materials and the "About this Mac" dialog.

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".