psychologist

n. expert in psychology, one who treats mental illnesses, one who studies mental processes

A psychologist is a person who studies psychology, the systematic investigation of the human mind, including behavior, cognition, and affect. Psychologists are usually categorized under a number of different fields, the most well-recognized being clinical psychologists, who provide mental health care and conduct substantive and applied research.
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Noun
1. a scientist trained in psychology
(hypernym) scientist, man of science
(hyponym) behaviorist, behaviourist

(n.)
One who is versed in, devoted to, psychology.
  

A professional concerned with the mind and behavior. Training in psychology leads to the masters and doctor of philosophy (PhD) degrees.