Attention-deficit hyperactivity disorder
Attention deficit hyperactivity disorder (ADHD or AD/HD or ADD) is a developmental disorder
Developmental disorder
Developmental disorders occur at some stage in a child's development, often retarding the development. These may include,psychological or physical disorders. The disorder is an impairment in the normal development of motor or cognitive skills that are developed before age 18 in which they are...

. It is primarily characterized by "the co-existence of attentional problems and hyperactivity, with each behavior occurring infrequently alone" and symptoms starting before seven years of age.

ADHD is the most commonly studied and diagnosed psychiatric disorder in children, affecting about 3 to 5 percent of children globally and diagnosed in about 2 to 16 percent of school aged children.