The study of mental and emotional disorders or maladaptive behaviors, or of mental phenomena such as dreams, hypnosis, and altered states or levels of consciousness. or psychopathology Branch of psychology. It is concerned with mental and emotional disorders (e.g., neurosis, psychosis, mental deficiency) and with certain incompletely understood normal phenomena (such as dreams and hypnosis). The chief tool used in classifying psychological disorders is the American Psychiatric Assn.'s Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV)