A professorship is the job itself — the post of professor at a university or college. When a school offers you a professorship, they are offering that seat, that title, and that work, not just calling you a nice name. Though actually, English is messy here. In American English, many people who teach at a university are called professor, so the everyday word is wider than this noun. In British English, professor is a high title, and a professorship is a rather special chair. Either way, the -ship word names the position, not the person walking around campus. It is a formal, academic word.