What is ADHD? According to the National Institute of Mental Health, the initials ADHD stand for attention-deficit/hyperactivity disorder. It's a brain disorder marked by an ongoing pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development. Some of the symptoms include impulsivity, hyperactivity and inattention. Conditions common in people who have ADHD include learning disabilities, anxiety disorder, conduct disorder, depression and substance abuse.