Autism is a neurodevelopmental disorder characterized by impaired social interaction, verbal and non-verbal communication, and restricted and repetitive behavior. Parents usually notice signs in the first two years of their child's life. The signs typically develop gradually, but some children with autism will reach their developmental milestones at a normal pace and then regress.

Autism is highly heritable, but the cause includes both environmental factors and genetic susceptibility. In rare cases, autism is strongly associated with agents that cause birth defects. Controversies surround other proposed environmental causes; for example, the vaccine hypotheses are biologically implausible and have been disproven in scientific studies. The diagnostic criteria require that symptoms become apparent in early childhood, typically before age three. Autism affects information processing in the 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 (ASDs), the other two being Asperger syndrome, which lacks delays in cognitive development and language, and pervasive developmental disorder, not otherwise specified (commonly abbreviated as PDD-NOS), which is diagnosed when the full set of criteria for autism or Asperger syndrome are not met.