Borderline personality disorder is a serious mental illness marked by unstable moods, behavior, and relationships. In 1980, the Diagnostic and Statistical Manual for Mental Disorders, Third Edition (DSM-III) listed borderline personality disorder as a diagnosable illness for the first time. Most psychiatrists and other mental health professionals use the DSM to diagnose mental illnesses.Because some people with severe borderline personality disorder have brief psychotic episodes, experts originally thought of this illness as atypical, or borderline, versions of other mental disorders. While mental health experts nowgenerally agree that the name "borderline personality disorder" is misleading, a more accurate term does not exist yet.