ADHD, or Attention-Deficit/Hyperactivity Disorder, is a neurodevelopmental disorder characterized by persistent patterns of inattention, hyperactivity, and impulsivity that interfere with functioning or development. It typically manifests in childhood and can continue into adulthood. The symptoms are divided into three categories: predominantly inattentive presentation, predominantly hyperactive-impulsive presentation, and combined presentation, which includes both inattentive and hyperactive-impulsive symptoms.