The history of it is... interesting and somewhat unfortunate. It at-least helps understand why we've got the mess we have now. The early history of the study of autism is muddled with the politics and hatred of WWII.
IIRC the current diagnostic system is 'Autism Spectrum Disorder' and then there's a level stated. 1: Requires Support; some trouble in social situations, rigid/inflexible behavior, stress during transitions 2: Requires Substantial Support; atypical social behavior, high interest in specific topics, noticable distress with change 3: Requires Very Substantial Support; severe communication deficits, repetitive behaviors, extreme distress when asked to switch tasks