Used as both an adjective and a noun in the context of autism.
In adjective form, it describes or refers to an aspect of autism. If a person has autism, they are an autistic person. If a behavior pattern, such as echolalia, is caused by autism, it is an autistic behavior.
In noun form, it always refers to a person with autism. Thus, I could be described as an autistic. For some reason, this is more common in English than the term autist.
Usage note: Proponents of person first language may object to the terms autistic person or an autistic, preferring person with autism or person who has autism. This is a highly controversial topic, however, and the vast majority of people with autism are not as politically correct as the non-autistic folks who came up with person first language. Many autists prefer autistic person or autistic just as strongly as person first language proponents prefer otherwise. Others simply don't care one way or the other, and still others prefer the person first version. If in doubt when speaking to an autistic person, ask how they prefer to be described. Chances are, as long as you are respectful of them and their preferences, it won't matter -- within reason -- what you call them.