California Code, Business and Professions Code - BPC § 2903

(a) No person may engage in the practice of psychology, or represent themselves to be a psychologist, without a license granted under this chapter, except as otherwise provided in this chapter.  The practice of psychology is defined as rendering or offering to render to individuals, groups, organizations, or the public any psychological service involving the application of psychological principles, methods, and procedures of understanding, predicting, and influencing behavior, such as the principles pertaining to learning, perception, motivation, emotions, and interpersonal relationships; and the methods and procedures of interviewing, counseling, psychotherapy, behavior modification, and hypnosis; and of constructing, administering, and interpreting tests of mental abilities, aptitudes, interests, attitudes, personality characteristics, emotions, and motivations.

(b) The application of these principles and methods includes, but is not restricted to, assessment, diagnosis, prevention, treatment, and intervention to increase effective functioning of individuals, groups, and organizations.

(c) Psychotherapy within the meaning of this chapter means the use of psychological methods in a professional relationship to assist a person or persons to acquire greater human effectiveness or to modify feelings, conditions, attitudes, and behaviors that are emotionally, intellectually, or socially ineffectual or maladaptive.