A severe pervasive developmental disorder with onset usually before three years of age and a biological basis related to neurologic or neurophysiologic factors.

It is characterized by qualitative impairment in reciprocal social interaction (e.g., lack of awareness of the existence of feelings of others, failure to seek comfort at times of distress, lack of imitation), in verbal and nonverbal communication, and in capacity for symbolic play, and by restricted and unusual repertoire of activities and interests.

Other characteristics sometimes include cognitive impairment, hyperactivitiy or hyporeactivity to certain stimuli, stereotypic behaviors, neurological abnormalities; such as, seizures or altered muscle tone, sleeping or eating pattern abnormalities, and severe behavioral problems. It is associated with several genetic conditions and prenatal and perinatal risk factors.