Borderline personality disorder (BPD) is a personality disorder defined in DSM-IV and described as a prolonged disturbance of personality function in a person (generally over the age of eighteen years, although it is also found in adolescents), characterized by depth and variability of moods. The disorder typically involves unusual levels of instability in mood; black and white thinking, or splitting; chaotic and unstable interpersonal relationships, self-image, identity, and behavior; as well as a disturbance in the individual's sense of self. In extreme cases, this disturbance in the sense of self can lead to periods of dissociation.

These disturbances can have a pervasive negative impact on many or all of the psychosocial facets of life. This includes difficulties maintaining relationships in work, home, and social settings. Self-harming is a marked symptom and even acts of attempted (or complete) suicide are possible, especially without proper care and effective therapy.

There is an ongoing debate among clinicians and patients worldwide about terminology and the use of the word borderline, and some have suggested that this disorder should be renamed. The ICD-10 manual has an alternative definition and terminology to this disorder, called Emotionally unstable personality disorder.

There is related concern that the diagnosis of BPD stigmatizes people, usually women, and supports pejorative and discriminatory practices. It is common for those suffering from borderline personality disorder and their families to feel compounded by a lack of clear diagnoses, effective treatments, and accurate information. This is true especially because of evidence that this disorder originates in the families of those who suffer from it and has a lot to do with Axis IV factors, rather than belonging strictly in Axis II. Conceptual, as well as therapeutic relief may be obtained through evidence that BPD is closely related to traumatic events during childhood and to Post-traumatic stress disorder (PTSD), about which much more is known.

In early 2008, at the request of BPD sufferers, the United States House of Representatives unanimously declared the month of May as Borderline Personality Disorder Awareness Month.