OS X, the desktop operating system which powers the Mac—“it's what makes a Mac a Mac,” says Apple—does not fit into the company's current OS naming scheme that has already given us watchOS, tvOS and iOS, but that might be about to change.
According to Portuguese-language site MacMagazine.com.br, developer Guilherme Rambo has discovered an interesting mention of 'macOS' in the “FUFlightViewController_macOS.nib” interface file within the FlightUtilities framework inside OS X code.