static in this case can be understood to mean simply "prior to (or separate from) runtime". In other words, it's based on what you can check _without_ running the code.

It's worth noting that nearly every statically-typed language currently in existence has two separate "type systems" - the static type system which is the formal type system offered to the programmer, plus a runtime type system enforced, at minimum, by the processor (e.g. you cannot divide by 0) that is ultimately different from the static type system. The 'question' in most cases is "how closely does the runtime type system match the static type system?" In many if not most languages, the answer is "not very".