description: An individual who specializes in psychological research, testing, and/or therapy. Psychology is the branch of science that deals with mental processes and behavior, composed of the following major fields: abnormal, clinical, comparative, counseling, developmental, educational, engineering, experimental, industrial, learning, motivation, perception, personality, physiological, psychometrics, school, and social psychology. Source: (1) Lexikon: Dictionary of Health Care Terms, Organizations and Acronyms for the Era of Reform, The Joint Commission on Accreditation of Healthcare Organizations, Oakbrook Terrace, Illinois: 1994, p. 654.