The act of identifying a disease or condition, mental or otherwise, through analysis of its symptoms and the use of various tests or procedures. In the United States, a commonly used source of diagnostic criteria for mental disorders is the DSM, or Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association.

Example: After administering several diagnostic tests for depression, interviewing her patient, and consulting the DSM, the psychologist concludes that the client is suffering from moderate clinical depression.