A neurodevelopmental disorder characterized by markedly impaired social interactions and verbal and nonverbal communication; narrow interests; and repetitive behavior. Manifestations and features of the disorder appear before age 3 but vary greatly across children according to developmental level, language skills, and chronological age. They may include a lack of awareness of the feelings of others, impaired ability to imitate, absence of social play, abnormal speech, abnormal nonverbal communication, and a preference for maintaining environmental sameness.