Autism is a disorder of neural development (Neurodevelopmental disorder) characterized by impaired social interaction (Interpersonal relationship) and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain (Human brain) by altering how nerve cells and their synapses connect and organize; how this occurs is not well understood. It is one of three recognized disorders in the autism spectrum, the other two being Asperger syndrome, which lacks delays in cognitive development and language, and Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS) (commonly abbreviated as PDD-NOS), which is diagnosed when the full set of criteria for autism or Asperger syndrome are not met.

Autism has a strong genetic basis, although the genetics of autism (Heritability of autism) are complex and it is unclear whether ASD is explained more by rare mutations, or by rare combinations of common genetic variants. In rare cases, autism is strongly associated with agents that cause birth defects (Teratology). Controversies (Controversies in autism) surround other proposed environmental causes (Causes of autism), such as heavy metals (Heavy metal (chemistry)), pesticides or childhood vaccines; the vaccine hypotheses are biologically implausible and lack convincing scientific evidence. The prevalence of autism is about 1-2 per 1,000 people; the prevalence of ASD is about 6 per 1,000, with about four times as many males as females. The number of people diagnosed with autism has increased dramatically since the 1980s, partly due to changes in diagnostic practice; the question of whether actual prevalence has increased is unresolved.

Parents usually notice signs in the first two years of their childs life. The signs usually develop gradually, but some autistic children first develop more normally and then regress (Regressive autism). Although early behavioral or cognitive intervention can help autistic children gain self-care, social, and communication skills, there is no known cure. Not many children with autism live independently after reaching adulthood, though some become successful. An autistic culture (Sociological and cultural aspects of autism) has developed, with some individuals seeking a cure and others believing autism should be tolerated as a difference and not treated as a disorder.