the macintosh operating system (mac os) is a program that allows a macintosh computer to operate and be used. mac os was first made by apple inc. in 1984. in those days mac os was called macintosh system software – which was shortened to system or ssw (system software). the term "mac os" first appeared in the splash screen for system 7.5.1, but was not adopted as the new name until the release of mac os 7.6.