What is autism? How is it defined?
Autism is also a spectrum disorder. In basic terms, this means you can be a little autistic or very autistic. At one end of the spectrum is Asperger Syndrome, sometimes called The Little Professor syndrome. At at the other end of the spectrum is the disorder that is most often called classic or Kannerian autism, which often includes profound developmental delays and challenges. In between are a variety of pervasive developmental disorders including Rett syndrome, Fragile X Syndrome, and pervasive developmental disorder not otherwise specified (PDD-NOS).
Not only is autism a spectrum disorder, but recent studies suggest that there may be more than one type of autism. While some autistic people have additional symptoms such as gastrointestinal issues, seizures and even mental illness, others have no such symptoms. The jury is still out on the question of "many autisms," so for now the term "autism spectrum" covers a wide set of differences and disabilities.
Sources:
Autism Society of America Website
Greenspan, Stanley. "The Child with Special Needs." C 1998: Perseus Books.
Romanowski, Patricia et al. "The OASIS Guide to Asperger Syndrome." C 2000: Crown Publishers, New York, NY.

