Systems design, insofar as I understand it, is difficult to define. It’s difficult to define not because no one has thought to define it — they have:
Systems design is the process of defining the architecture, product design, modules, interfaces, and data for a system to satisfy specified requirements.
Rather, I say it’s difficult to define because the descriptions I have read (like above) have done little to help me really understand what systems design is to the point that I could explain it in my own words to another person.
My mind tends to move toward graphs when I think about systems design. The system being the sum of the individual nodes and the edges which connect them. The output of a system for a given input would be the result of walking the graph from some point until reaching something defined as an end.