a mild form of amphetamine used in the treatment of attention-deficit hyperactivity disorder (attention-deficit/hyperactivity disorder) (ADHD), a condition that occurs primarily in children and is characterized by hyperactivity, inability to concentrate for long periods of time, and impulsivity. Ritalin, a trade-name drug, also has been effective for the treatment of other conditions such as narcolepsy. Although it acts as a stimulant in most people, Ritalin calms and focuses people with ADHD. Ritalin's mode of action is unknown, but researchers speculate that the drug reduces the symptoms of ADHD by increasing the amount and activity of a neurotransmitter in the brain.