Professionals think of autism as a "spectrum" disorder. This spectrum means a group of disorders with similar features. A child or an adult may have only mild symptoms, while another one may have very severe symptoms but they all have an autism spectrum disorder.
There is still a lot of disagreement and confusion among professionals about the terms however currently, the autism spectrum disorder (ASD) category includes:
1. Autistic disorder (also called "classic" autism)
3. Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS) or atypical autism
In some cases, professionals use a broader term, "PDD" that means Pervasive Development Disorders to describe autism. The PDD category includes five disorders: three disorders from the autism spectrum disorders listed above, plus Childhood Disintegrative Disorder and Rett's disorder.
The diagnosis of any of these five disorders is based upon the Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition (DSM-IV, 1994). DSM-IV is published by the American Psychiatric Association and is the main diagnostic reference of mental health professionals in the U.S. To read more about diagnosis, click here.