A penologist is a person who studies prisons and how we punish people who break the law. The study itself is called penology — so the -ist here is the specialist, not the subject. A penologist asks what prisons are for (to punish, to warn others, to help someone change), what life inside is like, and what actually works. It is a specialist, academic word. You will see it in university work, government reports, and serious news about crime and prisons, not in everyday chat. Though people sometimes mix it up, this is not a palaeontologist, who studies old bones and fossils; the two jobs could hardly be more different.