Between versions 7 and 10 it was officially MacOS [VERSION], before that it was referred to as system [VERSION], as in System 6 and so on. That was not an official name, it was the version number of the file `system` and was simply the system that ran on a Mac, because there was only one.
It got the official name MacOS when they allowed the selling of clones.
It got the official name MacOS when they allowed the selling of clones.