A seismologist is a scientist who studies earthquakes: how they start, how the shaking travels through the ground, and what that can tell us. The work, the field, is seismology; this word names the person who does it. You will hear it in news after a big quake — “seismologists say the centre was…” — and it feels like a job title, not slang. It is a formal word, but not a rare one in headlines. It does not mean the machine that records the shake (that is a seismograph), and it does not mean the study itself. If someone uses this word for the science, they have slipped; kindly put them back on the person.