I don't think so either, but I do think that leaving it just at "Unix is an operating system" is likely to generate a good more confusion along the lines of "wa

I don't think so either, but I do think that leaving it just at "Unix is an operating system" is likely to generate a good more confusion along the lines of "wait, I just have linux, where can I get unix?", "what do you mean linux is unix-like?", and of course "I thought I could use OSX for this". It should be strongly mentioned that "unix" best describes a class of operating systems including "proper" unix systems like OSX and the "unix-like" systems like linux. It's also important to divorce the students from that layman perspective of "operating systems are all that graphical shit I see on my screen". In fact, I think that might be the most important part.