Computation. So what is computable, algorithms, analysis of algorithms etc
Non of which you need a computer for.
An operating system in its self is an application of computation. It's like physics and a car. There is stuff in a operating system which is computer science, like how to schedule tasks etc but learning a specific os is not computer science.
Programming languages are computer science since they describe computation. Compilers are fuzzy
People have been doing computer science, long before computers existed.
I think at its core computer science is solving problems using mechanical steps(algorithms), although some people would say its even more general then that.