any one of several pervasive developmental disorders with an onset typically occurring during the preschool years. Each disorder is characterized by varying but often marked difficulties in communication and social interaction. The group includes autism, pervasive developmental disorders, Asperger's Disorder, and Rett's syndrome, among others. Substantial proportions of children with autistic spectrum disorders, excluding Asperger's syndrome, are reported to also have varying degrees of mental retardation and cognitive deficiencies. Also known as autism spectrum disorder.