Autism Spectrum Disorder. The terms ASD, autism, and PDD (Pervasive Developmental Disorder) are often used interchangeably. ASD is a “spectrum” disorder, in that people with autism can have very different symptoms. ASD covers a group of disorders with similar features. While one person on the spectrum may have mild symptoms, another person may have more significant symptoms. Currently, the ASD category includes: Autistic Disorder (sometimes referred to as “classic” autism), Rett’s Disorder or Rett Syndrome, Asperger’s Syndrome (or Disorder), Childhood Disintegrative Disorder, and Pervasive Developmental Disorder—Not Otherwise Specified (PDD-NOS). These neurobiological disorders affect a person’s ability to interact, communicate, relate, play, imagine, and learn.