An organist is a person who plays the organ — the big keyboard instrument you hear in churches, concert halls, and some old cinemas. Many organists play every week in church; some give concerts on huge pipe organs with ranks of pipes. The word is the ordinary, slightly formal name for that musician, the same pattern as pianist for piano. It is a job-and-art word, not slang. Though actually, sitting next to organ on the page can trick the eye: this organ is the musical one, not a heart or a liver, and an organist is not a doctor.