Borderline personality disorder is a mental illness that severely impacts a person’s ability to regulate their emotions. This loss of emotional control can increase impulsivity, affect how a person feels about themselves, and negatively impact their relationships with others."

Source : US National Institute of Mental Health (archived)

The first 'formal' definition of the disorder is thought to be from Aldoph Stern in 1938 (ref.) - though descriptions of patients with broadly similar symptoms go back as far as Ancient Greece.

The US NIMH (as linked above) describes the following symptoms :

People with borderline personality disorder may experience intense mood swings and feel uncertainty about how they see themselves. Their feelings for others can change quickly, and swing from extreme closeness to extreme dislike. These changing feelings can lead to unstable relationships and emotional pain.
People with borderline personality disorder also tend to view things in extremes, such as all good or all bad. Their interests and values can change quickly, and they may act impulsively or recklessly."

Other signs or symptoms may include:

• Efforts to avoid real or perceived abandonment, such as plunging headfirst into relationships—or ending them just as quickly.
• A pattern of intense and unstable relationships with family, friends, and loved ones.
• A distorted and unstable self-image or sense of self.
• Impulsive and often dangerous behaviors, such as spending sprees, unsafe sex, substance abuse, reckless driving, and binge eating. Please note: If these behaviors happen mostly during times of elevated mood or energy, they may be symptoms of a mood disorder and not borderline personality disorder.
• Self-harming behavior, such as cutting.
• Recurring thoughts of suicidal behaviors or threats.
• Intense and highly variable moods, with episodes lasting from a few hours to a few days.
• Chronic feelings of emptiness.
• Inappropriate, intense anger or problems controlling anger.
• Feelings of dissociation, such as feeling cut off from oneself, observing oneself from outside one’s body, or feelings of unreality.

BPD is currently estimated to be present in 1-3% of the general population, as well as in 10% of outpatients, 15-20% of inpatients, and 30-60% of patients with a diagnosed personality disorder, (ref.)

The causes of the disorder are currently unknown - but are thought likely to have some association with family history, 'poor parenting', brain structure, brain chemistry, environmental factors, social factors. Or some combination of those, and/or other as-yet-unknown causes.

Psychotherapy is the treatment of choice for BPD. Various approaches have been empirically supported in randomized controlled trials, including dialectical behavior therapy, mentalization-based therapy, transference-focused therapy, and schema therapy. No approach has proved to be superior to others.
[…]
No evidence is available consistently showing that any psychoactive medication is efficacious for the core features of BPD. For discrete and severe comorbid anxiety or depressive symptoms or psychotic-like features, pharmacotherapy may be useful"

Source : World Psychiatry, Volume 23, Issue 1, 2024 .

Note : The disorder - a.k.a. Emotionally Unstable Personality Disorder (EUPD) - is currently listed in the US DSM-5 ^{[ paywalled ]} as well as the World Health Organization's ICD-11 ( ref. )