A neurodevelopmental disorder. ASD symptoms are typically evident before a child is three years of age. The symptoms range from mild to severe—and individuals often have varied skills levels in different domains of functioning. Autism affects the person's overall development in two primary areas:
the presence of restricted, repetitive patterns of behavior, interests, or activities (e.g., repetitive motor movements, echolalia/repeating speech, idiosyncratic phrases, extreme distress at small changes/difficulty with transitions, strong attachment to unusual objects/topics, adverse or intense responses to sensory input)
Source: "Glossary of Terms Related to Neurodevelopmental Disabilities" by John Thorne, PhD, CCC-SLP.