1) A disorder beginning in childhood. It is marked by the presence of markedly abnormal or impaired development in social interaction and communication and a markedly restricted repertoire of activity and interest. Manifestations of the disorder vary greatly depending on the developmental level and chronological age of the individual. (DSM-IV)

Definition from: Unified Medical Language System (MeSH) at the National Library of Medicine A developmental disorder that appears by age three and that is variable in expression but is recognised and diagnosed by impairment of the ability to form normal social relationships, by impairment of the ability to communicate with others, and by stereotyped behaviour patterns especially as exhibited by a preoccupation with repetitive activities of restricted focus rather than with flexible and imaginative ones.

2) Autism is a developmental brain disorder characterised by impaired social interactions, communication problems, and repetitive behaviors. Symptoms usually appear before the age of three. The exact cause of autism is not known, however, it is likely influenced by genetics. Autism is one of a group of related developmental disorders called autism spectrum disorders (ASDs). Other ASDs include Asperger syndrome and Rett syndrome.