Borderline personality disorder (BPD) is a long-term mental illness. It is a type of cluster B personality disorder. People diagnosed with BPD are often very impulsive, and generally have a low self-esteem. Moods often change quickly. For these reasons, these people often have trouble keeping a stable relationship. Frequently, people with BPD also suffer from other conditions, such as clinical depression or they show self-harming behavior. Treating people with BPD is difficult, and is usually done using a combination of therapy and drugs.
C.H.Huges used the term "Borderland", to describe a number of conditions bordering mental health issues. Adolf Stern gave a description of some of the symptoms in 1938, and called them "border line group". People showed both conditions related to psychosis, and related to neurosis, so the term was thought fitting, at the time.