A medical doctor specializing in mental disorders. 'Alienist' is an archaic term.
He has worked as a psychiatrist for many years.
I see a psychiatrist.
More common term for 'psychiatrist'. 精神病医 is less frequent and may sound slightly more clinical or old-fashioned.
Psychologist (non-medical). 精神病医 is a medical doctor who can prescribe medication.
Compound of 精神 (mind, spirit), 病 (illness), and 医 (doctor).