Attention deficit hyperactivity disorder (ADHD) is a psychiatric disorder or neurobehavioral disorder characterized by significant difficulties either of inattention and/or hyperactivity and impulsiveness. Symptoms emerge before seven years of age. There are three subtypes of the disorder which consist of it being predominantly inattentive (ADHD-PI or ADHD-I), predominantly hyperactive-impulsive (ADHD-HI or ADHD-H), or the two combined (ADHD-C), which shows all three difficulties. Often people refer to ADHD-PI as "attention deficit disorder" (ADD), however, the latter has not been officially accepted since the 1994 revision of the DSM. ADHD impacts school-aged children and results in restlessness, acting impulsively, and lack of focus which may impair school performance. The most commonly diagnosed behavior disorder in young people, the Center for Disease Control and Prevention (CDC) reports that ADHD affects an estimated 9 percent of children aged 3-17 and 2-4 percent of adults. Although ADHD has its onset and is usually diagnosed in childhood, it is not a disorder limited to children - ADHD often persists into adolescence and adulthood and is frequently not diagnosed until later years.
For more information about ADHD, check out the National Institute of Mental Health.