Often abbreviated ASD.
This term encompasses the diagnosis of autism as well as related diagnoses such as Asperger's syndrome, childhood disintegrative disorder (Heller's syndrome), Rett's syndrome, and PDD-NOS (atypical autism). It can be synonymous with pervasive developmental disorder, although there are times when it is only used to mean three specific conditions (autism, Asperger syndrome, and PDD-NOS).
In conventional terms of what is known as the "triad of impairments", it refers to people who all have difficulty socializing in the usual way, difficulty communicating in verbal or nonverbal ways, and who are inflexible or repetitive in several areas of life. There is some controversy over whether this is a wholly accurate representation of autistic spectrum disorders.
The idea behind the spectrum is that lines cannot be as easily drawn between these conditions as the diagnostic criteria make them sound. Children who are initially diagnosed with autism might grow up to meet the criteria for Asperger's syndrome (Temple Grandin believes this has happened to her). Some children who initially meet the criteria for Asperger's syndrome even grow up to meet the criteria for autism. Children who are diagnosed with childhood disintegrative disorder show loss of certain skills after the age of four years, but some children who are diagnosed with autism show loss of the same skills in the same manner after two years. Some people carry the gene for but not the full expression of Rett's syndrome, and some of them are diagnosed with autism or a related condition. Many people in the same family might be diagnosed with different autistic conditions.
Some people view the spectrum as more or less linear. Classical autism lies on one end, Asperger's or PDD-NOS on the other. In some cases, it goes from low functioning autism, to PDD-NOS, to high functioning autism, to Asperger's syndrome. Other people disagree, saying that this is too simplistic and that spectrum is only an analogy for the way a number of conditions blend together in several quantitative and qualitative dimensions. The consensus is that these conditions are connected in some way, although people argue about exactly how.
Autistic spectrum disorder is also becoming a common diagnosis in its own right in some parts of the world, such as the United Kingdom and some European countries. When used this way, it encompasses what are known in the United States as Asperger Disorder, Autistic Disorder, and pervasive developmental disorder, not otherwise specified. In some places, though, it is used in the same way that PDD-NOS is sometimes used: As a statement that someone is "on the spectrum" but that the clinician is unable or unwilling to apply the autism or Asperger's criteria to them.
Sources:
Rett's Syndrome. http://www.asaoakland.org/retts.htm. Autism Society of America, Oakland County Chapter. Accessed 2004.
Wing, Lorna. The Autistic Spectrum: A Parent's Guide to Understanding Your Child. Berkeley: Ulysses Press, 2001.