Autism spectrum disorder (ASD) is a behaviorally defined
neurodevelopmental disorder associated with the presence of social-communication
deficits and restricted and repetitive behaviors. In the latest
conceptualization of ASD, these two behavioral dimensions represent the core
defining features of ASD, whereas associated dimensions, such as intellectual
and language ability, provide a means for describing the ASD heterogeneity. In
addition, the characterization of ASD subgroups, defined by the presence of
known medical, genetic, or other psychiatric disorders, furthers our
understanding of ASD heterogeneity. This paper reviews the history of autism,
describes its core defining features, and provides an overview of the clinically
and etiologically relevant subgroups that add to the complexity of this
condition.