Autism, formally referred to as Autism Spectrum Disorder (ASD) in the US and Autism Spectrum Condition (ASC) in other parts of the world, and sometimes referred to as the autism spectrum, is a neurodevelopmental disorder characterized by deficits in social communication and social interaction, and repetitive or restricted patterns of behaviors, interests, or activities, which can include hyper- and hyporeactivity to sensory input.

Autism is a spectrum disorder, meaning it can manifest very differently in each person. Because of this, there is wide variation in the support needs of autistic people, for example some are nonspeaking, while others have proficient spoken language.

While psychiatry has traditionally classified autism as a neurodevelopmental disorder, the autism rights movement and some researchers see autism as part of neurodiversity, the natural diversity in human thinking and experience, with strengths, differences, and weaknesses. Autistic people still have a disability, but need to be accommodated, rather than cured. This view of the condition has led to significant controversy among those who are autistic alongside advocates, practitioners, and charities.

There are many theories about what causes autism; it is highly heritable and believed to be mainly genetic, but many genes are involved, and environmental factors may also be relevant. The syndrome frequently co-occurs with other conditions, including attention deficit hyperactivity disorder, epilepsy, and intellectual disability. Disagreements continue about questions such as what should be included as part of the diagnosis, whether there are meaningful subtypes of autism, and the significance of autism-associated traits in the wider population. The combination of broader criteria and increased awareness has led to a trend of steadily increasing estimates of autism prevalence, causing a misconception that there is an autism epidemic and perpetuating the myth that it is caused by vaccines.

Although there is no cure for autism, early behavioral interventions can help children with speech delays gain self-care, social, and language skills. Independent living is unlikely in those with higher support needs, so intervention for them requires finding and learning alternative modes of communication. It is well-established that the atypical antipsychotic drugs risperidone and aripiprazole alleviate the associated symptoms of irritability.