Psychologist (noun)

1. A person who is trained in psychology and is skilled in the study of the mind and behavior.
2. A professional who specializes in the study of the mind, brain, and behavior.
3. A person who practices psychology, the scientific study of human behavior and mental processes.

Origin:

From psychology + -ist.

Examples:

1. He is a clinical psychologist.
2. She is a developmental psychologist.
3. The psychologist helped the patient to understand their behavior.
4. The psychologist's report was important in the trial.
5. The psychologist's advice was very useful.