Attention deficit hyperactivity disorder: A disorder in which a person is unable to control behavior due to difficulty in processing neural stimuli, accompanied by an extremely high level of motor activity. Abbreviated ADHD. ADHD can affect children and adults, but it is easiest to perceive during schooling. A child with ADHD may be extremely distractible, unable to remain still, and very talkative. ADHD is diagnosed by using a combination of parent and/or patient interview, observation of the patient, and sometimes use of standardized screening instruments. Treatments include making adjustments to the environment to accommodate the disorder, behavior modification, and the use of medications. Stimulants are the most common drugs used, although certain other medications can be effective.