Autism, also known as autism spectrum disorder (ASD), is a condition characterized by differences or difficulties in social communication and interaction, a need or strong preference for predictability and routine, sensory processing differences, focused interests, and repetitive behaviors. Characteristics of autism are present from early childhood and the condition typically persists throughout life. Autism is classified as a neurodevelopmental disorder, and a formal diagnosis requires professional assessment that these characteristics cause significant challenges in daily life beyond what would be expected given a person's age and social environment. Because autism is a spectrum disorder, presentations vary and support needs range from minimal to being non-speaking or needing 24-hour care.

Autism diagnoses have risen since the 1990s, largely because of broader diagnostic criteria, greater awareness, and wider access to assessment. Changing social demands may also play a role. The World Health Organization estimates that about 1 in 100 children were diagnosed between 2012 and 2021, noting an increasing trend. Surveillance studies suggest a similar share of the adult population would meet diagnostic criteria if formally assessed. This rise has fueled anti-vaccine activists' disproven claim that vaccines cause autism, based on a fraudulent 1998 study that was later retracted. Autism is highly heritable and involves many genes, while environmental factors appear to play a smaller, mainly prenatal role. Boys are diagnosed several times more often than girls, and conditions such as anxiety, depression, attention deficit hyperactivity disorder (ADHD), epilepsy, and intellectual disability are more common among autistic people.

There is no cure for autism. Several autism therapies aim to improve self-care, social, and language skills. Reducing environmental and social barriers helps autistic people participate more fully in education, employment, and other aspects of life. No medication addresses the core features of autism, but some are used to help manage commonly co-occurring conditions, such as anxiety, depression, irritability, ADHD, and epilepsy.

Autistic people are found in every demographic group and, with appropriate supports that promote independence and self-determination, can participate fully in their communities and lead meaningful, productive lives. The idea of autism as a disorder has been challenged by the neurodiversity framework, which frames autistic traits as a healthy variation of the human condition. This perspective, promoted by the autism rights movement, has attracted increasing research attention, but remains a subject of debate and controversy among autistic people, advocacy groups, healthcare providers, and charities.

The DSM-5 and ICD-11 are the two main diagnostic manuals in use today, and both define autism as a neurodevelopmental disorder with a spectrum of highly varied presentations. The spectrum concept signals diversity rather than a simplistic range from mild to severe. Before the DSM-5 (2013) and ICD-11/ICD-11 CDDR (2019/2024), autism fell within a broader pervasive developmental disorder category that included labels such as Asperger syndrome and classic autism (also called childhood autism or Kanner syndrome). Because these diagnoses overlapped, the manuals unified them under autism spectrum disorder (ASD).

Since 1980, the committees behind both manuals have aimed for greater convergence, incorporating biological research while keeping behavior-based criteria. DSM-5 specifies three levels of support needs, from level 1 to level 3. ICD-11 instead records whether the person has co-occurring intellectual disability or language impairment.

Current research questions whether existing criteria capture the full phenomenon, prompting proposals for prototype descriptions, transdiagnostic biological markers, or separate consideration of common behavioral traits and rare genetic or environmental factors. Some proposed alternatives to the disorder-focused spectrum model deconstruct autism into separate phenomena: a non-pathological spectrum of behavioral traits in the population, and the effect of rare genetic mutations and environmental factors influencing neurodevelopmental and psychological conditions. Clinical and policy guidance in Australia, the United States, and the United Kingdom now promotes neurodiversity-affirming language—for example, saying "characteristics" instead of "symptoms" and avoiding words such as "cure".

The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), released in 2022, is the current version of the DSM. Its fifth edition – DSM-5, released in May 2013 – was the first to define ASD as a single diagnosis, combining the previously distinct diagnoses of classic autism, Asperger syndrome, childhood disintegrative disorder, and pervasive developmental disorder not otherwise specified (PDD-NOS). This is still the case in the DSM-5-TR.^{[non-primary source needed]}