Attention deficit hyperactivity disorder: A family of related chronic neurobiological disorders that interfere with an individual's capacity to:

• regulate activity level (hyperactivity),
• inhibit behavior (impulsivity), and
• attend to tasks (inattention) in developmentally appropriate ways. The term "attention deficit hyperactivity disorder" is abbreviated and usually referred to as ADHD.

The core symptoms of ADHD include an inability to sustain attention and concentration, developmentally inappropriate levels of activity, distractibility, and impulsivity.

Children with ADHD have functional impairment across multiple settings including home, school, and peer relationships. ADHD has also been shown to have long-term adverse effects on academic performance, vocational success, and social-emotional development. Children with ADHD experience an inability to sit still and pay attention in class and the negative consequences of such behavior. They experience peer rejection and engage in a broad array of disruptive behaviors. Their academic and social difficulties have far-reaching and long-term consequences. These children have higher injury rates. As they grow older, children with untreated ADHD, in combination with conduct disorders, experience drug abuse, antisocial behavior, and injuries of all sorts. For many individuals, the impact of ADHD continues into adulthood.

Physical abnormalities of the brain have been detected in ADHD. Using high resolution MRI and surface-based computational image analytical techniques, it was found that people with ADHD have reductions in total brain volume and in the volumes of specific areas of the brain. These brain abnormalities may contribute to or even account for the symptoms of ADHD.