Could you explain what is a "personality disorder"?

Personality Disorder: A personality disorder is an enduring, pervasive, and inflexible pattern of inner experience and outer behavior that differs markedly from one's culture, begins in adolescence or early adulthood, and causes distress in important areas of life (relationships, work, etc). There are multiple types which may blend into each other, but are diagnosed according to the person's most prominent features.

Clusters of traits: Usually diagnosed at 18+ (as an individual is still developing and finding their identity before then), personality disorders are made up of long -enduring, stable, clusters of traits. These traits are often maladaptive and can cause distress to the individuals and/or those involved with the individual.