Borderline personality disorder is classified in DSM-V as instability of self-image, personal
goals, interpersonal relationships, and affects, accompanied by impulsivity, risk taking,
and/ or hostility. The disorder is conceptually similar to a disorder classified in ICD-10 as emotionally unstable personality disorder. The term ‘borderline’ was introduced by the American psychoanalyst Adolph Stern in 1938, referring to patients who did not fit into either the category of neurosis or psychosis.

Source:

Stern, Adolph (1938), “Psychoanalytic Investigation of and Therapy in the Border Line Group of Neuroses”, Psychoanalytic Quarterly 7:467–489.

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