Classic autism—also known as childhood autism, autistic disorder, or Kanner's syndrome—is a formerly diagnosed neurodevelopmental disorder first described by Leo Kanner in 1943. It is characterized by atypical and impaired development in social interaction and communication as well as restricted and repetitive behaviors, activities, and interests. These symptoms first appear in early childhood and persist throughout life.

Classic autism was last recognized as a diagnosis in the DSM-IV and ICD-10, and has been superseded by autism-spectrum disorder in the DSM-5 (2013) and ICD-11 (2022). Globally, classic autism was estimated to affect 24.8 million people as of 2015^[update].

Autism is likely caused by a combination of genetic and environmental factors, with genetic factors thought to heavily predominate. Certain proposed environmental causes of autism have been met with controversy, such as the vaccine hypothesis that, although disproved, has negatively impacted vaccination rates among children.

Since the DSM-5/ICD-11, the term "autism" more commonly refers to the broader autism spectrum.

Autism is a highly variable neurodevelopmental disorder whose symptoms first appear during infancy or childhood, and generally follow a steady course without remission. Autistic people may be severely impaired in some respects but average, or even superior, in others. Overt symptoms gradually begin after the age of six months and become established by age two or three years. Some autistic children experience regression in their communication and social skills after reaching developmental milestones at a normal pace. It was said to be distinguished by a characteristic triad of symptoms: impairments in social interaction, impairments in communication, and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis. Individual symptoms of autism occur in the general population and appear not to highly be associated, without a sharp line separating pathologically severe from common traits.

Autistic people have social impairments and often lack the intuition about others that many people take for granted. Unusual social development becomes apparent early in childhood. Autistic infants show less attention to social stimuli, smile and look at others less often, and respond less to their own name. Autistic toddlers differ more strikingly from social norms; for example, they have less eye contact and turn-taking, and do not have the ability to use simple movements to express themselves, such as pointing at things. Three- to five-year-old autistic children are less likely to exhibit social understanding, approach others spontaneously, imitate and respond to emotions, communicate nonverbally, and take turns with others. However, they do form attachments to their primary caregivers. Most autistic children displayed moderately less attachment security than neurotypical children, although this difference disappears in children with higher mental development or less pronounced autistic traits. Children with high-functioning autism have more intense and frequent loneliness compared to non-autistic peers, despite the common belief that autistic children prefer to be alone. Making and maintaining friendships often proves to be difficult for autistic people. For them, the quality of friendships, not the number of friends, predicts how lonely they feel. Functional friendships, such as those resulting in invitations to parties, may affect the quality of life more deeply.

Differences in communication may be present from the first year of life, and may include delayed onset of babbling, unusual gestures, diminished responsiveness, and vocal patterns that are not synchronized with the caregiver. In the second and third years, autistic children have less frequent and less diverse babbling, consonants, words, and word combinations; their gestures are less often integrated with words. Autistic children are less likely to make requests or share experiences, and are more likely to simply repeat others' words (echolalia) or reverse pronouns. Deficits in joint attention may be present — for example, they may look at a pointing hand instead of the object to which the hand is pointing. Autistic children may have difficulty with imaginative play and with developing symbols into language. It is also thought that autistic and non-autistic adults produce different facial expressions, and that these differences could contribute to bidirectional communication difficulties.

Autistic individuals can display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R) categorizes as follows.