Autistic slang, short for autistics and cousins. When autistic people started to talk to each other on the net and through RL support groups, they discovered some people who had a lot in common with them, but weren't actually autistic. These people usually had other neurological conditions -- hydrocephalus, Tourette's syndrome, attention deficit disorder, specific learning difficulties, and so forth -- that, in these specific people, created perceptual or social traits similar to those of autistic people. Autistic people found them easier to communicate and share interests with than the
average neurotypical. Thus, these people came to be called cousins, because they were
related to autism, but were not (so the metaphor went) immediate relatives. Their neurological conditions became known as cousin conditions. In common usage, AC
encompasses both autistic people and cousins, and is used as the opposite to NT. It can be used as a noun or adjective.
Example usage: "Most of my friends are ACs, because they don't pressure me to act normal the way most NTs do."